Abstract
Purpose
The aim of this study was to identify factors associated with double embryo implantation following double embryo transfer (DET) during assisted reproductive technology (ART) procedures and to evaluate the implications of findings in selecting candidates for elective single embryo transfer (eSET).
Methods
Factors predicting double embryo implantation, defined as embryo transfers with two or more heartbeats on 6-week ultrasound following DET, were assessed using the US National ART Surveillance System data from 2000 to 2012 (n = 1,793,067 fresh, autologous transfers). Adjusted risk ratios (aRRs) were estimated after stratifying by prognosis. Favorable prognosis was defined as first-time ART with supernumerary embryo(s) cryopreserved. Average prognosis was defined as first-time ART without supernumerary embryo(s) cryopreserved, prior unsuccessful ART with supernumerary embryo(s) cryopreserved, or prior ART with previous birth(s) conceived with ART or naturally. Rates and factors associated with double embryo implantation were compared with single embryo implantation following DET among both prognosis groups.
Results
Double embryo implantation was positively associated with blastocyst (versus cleavage) transfer in favorable (aRR = 1.58 (1.51–1.65)) and average (aRR = 1.67 (1.60–1.75)) prognosis groups and negatively associated with age >35 years in both prognosis groups. For average prognosis patients, double embryo implantation was associated with retrieving >10 oocytes (aRR = 1.22 (1.18–1.24)).
Conclusions
Regardless of prognosis, patients aged <35 years with blastocyst-stage embryos and average prognosis patients from whom >10 oocytes were retrieved may be good candidates for eSET. Physicians may consider using these data to counsel patients on eSET, which would reduce multiple gestations and associated complications.
Keywords: Double embryo transfer, In vitro fertilization, Double embryo implantation, Multiple birth pregnancy, Elective single embryo transfer
Introduction
Fertility treatments, both assisted reproductive technology (ART) and non-ART, have led to an increase in multiple gestation pregnancies over the past 40 years [1, 2]. Due to frequent transfers of multiple embryos, almost half of ART-conceived infants in the USA are born as part of multiple gestations [3]. Multiple birth pregnancies, even twin gestations, increase both maternal and infant risks for adverse perinatal outcomes [1, 4, 5]. Maternal rates of preeclampsia, preterm premature rupture of membranes, and cesarean delivery are higher in multiple gestation pregnancies compared to singleton pregnancies [6]. Similarly, neonatal rates of prematurity, low birth weight, and associated morbidity and mortality are higher in infants born as multiples than singletons [3, 5, 7]. A successful ART outcome is increasingly defined as a healthy singleton [7, 8].
In effort to improve perinatal outcomes following ART, guidelines developed by the American Society of Reproductive Medicine (ASRM) and the Society for Assisted Reproductive Technology (SART) encourage the use of elective single embryo transfer (eSET), defined as the transfer of one embryo when more than one high-quality embryo is available, in certain patient groups such as younger patients with a favorable prognosis [8–10]. Although monozygotic twinning can occur, eSET in appropriate groups reduces the potential for multiple gestation pregnancies and increases the potential for a good perinatal outcome, defined by many as a singleton, term (≥37 weeks) infant with birth weight ≥2500 g [8, 11]. Characteristics associated with a good perinatal outcome following eSET include female patients <38 years of age, diagnosis of male factor infertility, day 5 blastocyst embryo transfer (ET), and having >3 embryos available for cryopreservation [10]. Although the frequency of eSET has increased in recent years, in 2013, only 21 % of transfers among women <35 years of age and 13 % of all transfers among women 35–37 years of age were eSET [5].
Despite guidelines and reports promoting eSET, the rate of double embryo transfer (DET) and resulting multiple births remains high [5]. The overall rate of DET has increased over time, from 32 % of ETs in 2002 to 56 % of ETs in 2013, coinciding with fewer higher-order (three or more) ETs [2, 5]. Many providers and families still choose to transfer two embryos, even with a favorable prognosis, to maximize the chance of live birth with a single treatment. In the instances where two embryos are transferred and both embryos are implanted, also known as double embryo implantation, there may be a missed opportunity for having a successful eSET, in which the infant is more likely to be born at term and an appropriate size for gestational age. A previous study by Hunault and colleagues similarly used DET resulting in multiple gestation pregnancy to develop a patient selection prediction model for eSET, concluding that younger age and embryo quality were the best predictors [12]. Only a few small studies have examined factors associated with double embryo implantation following DET. Many of these studies showed that double embryo implantation was associated with younger age [12–17], a higher number of retrieved oocytes [12, 13], and higher embryo quality [12, 15–18]. Given the limited information and small sample sizes on double embryo implantation following DET, we analyzed data from Centers for Disease Control and Prevention’s (CDC’s) National ART Surveillance System (NASS) to estimate the trends in rates of single embryo implantation following eSET and non-eSET, double embryo implantation following DET, triple embryo implantation following triplet embryo transfer (TET), and quadruple or higher embryo implantation following quadruple or higher embryo transfer (QET+), We assessed predictors of double embryo implantation following DET to further identify characteristics of potential good candidates for successful eSET. Given our large sample, we were able to stratify our population by prognosis to better assess factors that may be predictive of double embryo implantation, thus providing more information to better counsel patients on eSET who may otherwise chose DET.
Materials and methods
Data
Our data were derived from the NASS, a data reporting system established in accordance with the Fertility Clinic Success Rates and Certification Act of 1992, a federal law that requires all ART clinics in the USA to report data on each ART cycle [19]. Reported data include patient demographics, infertility diagnosis, clinical information, and outcome data for each ART cycle. The NASS includes data from approximately 97 % of all ART cycles performed in the USA [5]. We used data from 2000 to 2012 and restricted our analysis to fresh, non-donor ART embryo transfers. ART embryo transfers using zygote intrafallopian transfer or gamete intrafallopian transfer were excluded from analysis.
Double embryo implantation was defined as DET resulting in two or more heartbeats on 6-week ultrasound. We also examined embryo transfers where eSET and non-eSET resulted in one or more heartbeats on 6-week ultrasound, TET resulted in three or more heartbeats on 6-week ultrasound, and quadruple embryo transfer (QET) resulted in four or more heartbeats on 6-week ultrasound. Instances where more than four embryos were transferred were combined with the QET group (QET+).
We examined the rate of double embryo implantation following DET by age and prognosis. The favorable prognosis population was defined as women who underwent their first ART embryo transfer and had one or more supernumerary embryo(s) cryopreserved, which is a proxy for embryo quality. The less favorable prognosis population was defined as women who had previous ART embryo transfer(s), no previous live births, and no supernumerary embryos cryopreserved. The remaining population was classified as average prognosis and included women who (1) underwent their first ART embryo transfer and had no supernumerary embryos cryopreserved, (2) had previous ART embryo transfer(s) and no previous live birth(s) but had one or more supernumerary embryo(s) cryopreserved, or (3) had previous ART embryo transfer(s) and previous live birth(s) conceived with ART or naturally. A previous successful ART embryo transfer would be categorized as favorable prognosis by ASRM/SART; however, since the NASS does not distinguish live birth resulting from previous ART or from natural conception, we categorized this population as average prognosis. Given the small number of women with less favorable prognosis who had DET resulting in double embryo implantation (n = 4,748, 5.9 % of double embryo implantation population), we focused on the groups with favorable and average prognosis, who would be more suitable targets for reducing DET.
Study factors and outcome characteristics
To evaluate the factors associated with double embryo implantation following DET, we first assessed a number of characteristics among DETs resulting in double embryo implantation and compared them with DETs resulting in single embryo implantation. We restricted the analysis to women who became pregnant after DET to assess the differences between embryo transfers resulting in single embryo implantation and embryo transfers resulting in double embryo implantation. The characteristics assessed were maternal age, race/ethnicity, infertility diagnosis, number of prior pregnancies, number of prior spontaneous abortions, year of embryo transfer, embryo stage at transfer, number of oocytes retrieved, use of intracytoplasmic sperm injection (ICSI), use of assisted hatching, and use of preimplantation genetic diagnosis (PGD). PGD data were not collected in the NASS until 2004. For the race/ethnicity variable, approximately 40 % of the data were missing overall, so we created another/unknown race category. Since patients may have more than one infertility diagnosis, diagnosis categories were not mutually exclusive. We compared embryo transfers started during 2000–2004 and those started during 2005–2012 to evaluate the potential impact of the ASRM/SART guidelines that promoted eSET, which were first published on September 2004 [20].
Statistical analysis
SAS statistical software version 9.3 (Cary, NC) was used to conduct all analyses. To calculate the annual rate of embryo transfers where all transferred embryos were implanted among all ART transfers, the frequency of embryo transfers where all transferred embryos were implanted of each transfer quantity was divided by the frequency of that transfer quantity in one reporting year, i.e., frequency of DET with double embryo implantation in 2000 divided by the frequency of DET in 2000. We used the Cochran-Armitage trend test to assess trends in the rate of embryo transfers where all transferred embryos were implanted during the study period. Pearson’s two-tailed chi-square tests were used to compare the distribution of maternal and embryo transfer characteristics between the DET group with double embryo implantation and the DET group with single embryo implantation within each prognosis group. Log-binomial models were used to calculate the unadjusted and adjusted risk ratios (aRRs) for the association between patient and embryo transfer characteristics and double embryo implantation. Our model was restricted to day 2/3 cleavage transfers and day 5/6 blastocyst transfers, which represented 97.7 % of all transfers. The model included maternal age, infertility diagnosis, number of prior pregnancies, number of prior spontaneous abortions, stage of transfer (days 2/3 and 5/6 only), number of oocytes retrieved, ICSI, and assisted hatching. The model controlled for year of transfer as a continuous variable and for clinic cluster using generalized estimating equations to account for differing laboratory procedures and clinical techniques at each clinic. Since 40 % of the race/ethnicity and 20 % of PGD data were unknown or missing, we compared the findings for the adjusted models with and without these variables and subsequently removed race/ethnicity and PGD from the model since the differences in the effect estimates were minimal.
Ethical approval
The study was approved by CDC’s institutional review board.
Results
Overall, there were 1,793,067 ART cycles included in the NASS from 2000 to 2012, of which 1,234,871 used fresh, non-donor embryos and 998,579 (80.9 %) proceeded to transfer at least one embryo. During the study period, 11.4 % of transfers were SET, 44.2 % of transfers were DET, 27.1 % of transfers were TET, and 17.3 % of transfers were QET+.
Since 2000, the rates of single embryo implantation following eSET and double embryo implantation following DET increased, while the rate of non-elective single embryo implantation following SET remained the same and the rates of triple embryo implantation following TET and quadruple or higher embryo implantation following QET+ decreased (Fig. 1, p for all trends <0.0001). Elective single embryo implantation following SET increased from 1.5 % of transfers in 2000 to 24.7 % of transfers in 2012. Non-elective single embryo implantation following SET increased slightly then decreased back to baseline from 10.2 % of transfers in 2000 to 10.0 % of transfers in 2012. Double embryo implantation following DET increased from 13.4 % of transfers in 2000 to 18.1 % of transfers in 2012. Triple embryo implantation following TET decreased from 3.9 % of transfers in 2000 to 2.1 % of transfers in 2012, while quadruple embryo implantation following QET+ decreased from 0.5 % of transfers in 2000 to 0.02 % of transfers in 2012.
Fig. 1.
Trends in elective and non-elective single, double, triple, and quadruple or higher implantation rates, by the number of embryos transferred, USA, 2000–2012. a Elective and non-elective single embryo implantation following single embryo transfer, double embryo implantation following double embryo transfer, triple embryo implantation following triple embryo transfer, and quadruple or higher embryo implantation following quadruple or higher embryo transfer. b Data restricted to fresh, non-donor embryos. c p value for trends <0.0001
From 2000 through 2012, 79,813 DETs resulted in double embryo implantation for all prognosis groups. Within each age group (<35, 35–37, 38–40, and ≥41), women with a favorable prognosis had the highest rate of double embryo implantation following DET, and women in average and less favorable prognosis groups had lower rates (Fig. 2). Within each prognosis group, younger women had higher rates of double embryo implantation than women in older age groups. Women in the favorable prognosis group with age 35 to 37 years had a higher rate of double embryo implantation (22.1 %) compared to women in the average prognosis group with age less than 35 years (18.0 %). Among women with a favorable prognosis who had at least one embryo implant after DET, 37,491 (43.3 %) had double embryo implantation and 49,123 (56.7 %) had a single embryo implantation (Table 1). Among women with an average prognosis who had at least one embryo implant after DET, 34,574 (34.2 %) resulted in double embryo implantation and 66,555 (65.8 %) had single embryo implantation. For both favorable and average prognosis groups, the distribution of most maternal and clinical characteristics differed significantly between the double embryo implantation group and the single embryo implantation group (p < 0.05, Table 1). For example, a higher percentage of blastocyst transfers in both prognosis groups resulted in double embryo implantation than in single embryo implantation (p < 0.0001 for favorable and average prognosis). A significant difference in the distribution of uterine factor infertility, unexplained infertility, and number of prior pregnancies was only detected for the group with average prognosis.
Fig. 2.
Rate of double embryo implantation in double embryo transfer, stratified by age and prognosis, USA, 2000–2012. a Data restricted to fresh, non-donor embryos. b The favorable prognosis population was defined as those who underwent their first ART embryo transfer and had one or more supernumerary embryo(s) cryopreserved. The less favorable prognosis population was defined as those who had previous ART embryo transfer(s), no previous live births, and no supernumerary embryos cryopreserved. The remaining population was classified as average prognosis and included those who (1) underwent their first ART embryo transfer and had no supernumerary embryos cryopreserved, (2) had previous ART embryo transfer(s) and no previous lives birth(s) but had one or more supernumerary embryo(s) cryopreserved, or (3) had previous ART embryo transfer(s) and previous live birth(s)
Table 1.
Characteristics of double embryo transfers with double embryo implantation compared to double embryo transfers with single embryo implantation, stratified by prognosis, USA, 2000–2012
| Characteristic | Favorable prognosis | Average prognosis | ||||
|---|---|---|---|---|---|---|
| DET with double embryo implantation (%) | DET with single embryo implantation (%) | p value | DET with double embryo implantation (%) | DET with single embryo implantation (%) | p value | |
| 37,491 (43.3) | 49,123 (56.7) | 34,574 (34.2) | 66,555 (65.8) | |||
| Maternal age (years) | ||||||
| <35 | 29,312 (78.2) | 35,389 (72.0) | <0.0001 | 24,190 (70.0) | 40,095 (60.2) | <0.0001 |
| 35–37 | 6364 (17.0) | 9791 (19.9) | 7485 (21.6) | 16,075 (24.2) | ||
| 38–40 | 1700 (4.5) | 3565 (7.3) | 2596 (7.5) | 8248 (12.4) | ||
| ≥41 | 115 (0.3) | 384 (0.8) | 303 (0.9) | 2137 (3.2) | ||
| Race/ethnicity | ||||||
| Non-Hispanic white | 18,390 (49.1) | 23,275 (47.4) | <0.0001 | 16,612 (48.1) | 31,048 (47.4) | <0.001 |
| Non-Hispanic black | 1258 (3.4) | 1836 (3.7) | 1040 (3.0) | 2107 (3.2) | ||
| Asian/Pacific Islander | 1777 (4.7) | 2641 (5.4) | 1677 (4.9) | 3685 (5.5) | ||
| Hispanic | 2036 (5.4) | 2575 (5.2) | 1552 (4.5) | 2941 (4.4) | ||
| Other race/unknowna | 14,030 (37.4) | 18,796 (38.3) | 13,656 (39.5) | 26,696 (40.1) | ||
| Infertility diagnosisb | ||||||
| Tubal factor | 7006 (18.9) | 9333 (19.0) | 0.24 | 6452 (18.7) | 12,109 (18.2) | 0.07 |
| Endometriosis | 4705 (12.6) | 6211 (12.6) | 0.68 | 4580 (13.3) | 8753 (13.2) | 0.67 |
| Uterine factor | 1300 (3.5) | 1776 (3.6) | 0.24 | 1271 (3.7) | 2662 (4.0) | 0.01 |
| Ovulatory disorder | 8232 (22.0) | 9808 (20.0) | <0.0001 | 6981 (20.2) | 11,399 (17.1) | <0.0001 |
| Diminished ovarian reserve | 1714 (4.6) | 2715 (5.5) | <0.0001 | 2666 (7.7) | 7442 (11.2) | <0.0001 |
| Male factor | 14,588 (38.9) | 19,646 (40.0) | <0.01 | 14,391 (41.6) | 26,961 (40.5) | <0.001 |
| Unexplained | 4009 (10.7) | 5228 (10.6) | 0.81 | 3937 (11.4) | 8120 (12.2) | <0.001 |
| Number of prior pregnancies | ||||||
| 0 | 22,568 (60.3) | 29,378 (60.0) | 0.06 | 14,816 (42.9) | 28,887 (43.5) | <0.0001 |
| 1 | 7955 (21.3) | 10,303 (21.0) | 10,224 (29.6) | 18,857 (28.4) | ||
| ≥2 | 6880 (18.4) | 9327 (19.0) | 9476 (27.5) | 18,706 (28.1) | ||
| Number of prior spontaneous abortions | ||||||
| 0 | 29,433 (78.5) | 38,224 (77.8) | 0.01 | 24,953 (34.4) | 47,635 (71.6) | <0.01 |
| 1 | 5406 (14.4) | 7266 (14.8) | 6503 (34.3) | 12,448 (18.7) | ||
| ≥2 | 2650 (7.1) | 3628 (7.4) | 3117 (32.5) | 6469 (9.7) | ||
| Year cycle treatment started | ||||||
| 2000–2004 | 8537 (22.8) | 12,026 (24.5) | <0.0001 | 7886 (22.8) | 15,834 (23.8) | <0.001 |
| 2005–2012 | 28,954 (77.2) | 37,097 (75.5) | 26,688 (77.2) | 50,721 (76.2) | ||
| Stage of transfer | ||||||
| Cleavage (day 2/3) | 11,670 (31.1) | 21,049 (42.8) | <0.0001 | 13,715 (39.7) | 35,528 (53.4) | <0.0001 |
| Blastocyst (day 5/6) | 25,098 (66.9) | 27,011 (55.0) | 20,141 (58.3) | 29,471 (44.3) | ||
| Other | 723 (1.9) | 1063 (2.2) | 718 (2.1) | 1556 (2.3) | ||
| Number of oocytes retrieved | ||||||
| 1–10 | 6454 (17.2) | 9731 (19.8) | <0.0001 | 11,553 (33.4) | 29,146 (43.8) | <0.0001 |
| 11–15 | 10,812 (28.8) | 14,394 (29.3) | 9978 (28.9) | 17,497 (26.3) | ||
| ≥16 | 20,225 (54.0) | 24,998 (50.9) | 13,043 (37.7) | 19,912 (29.9) | ||
| Use of intracytoplasmic sperm (ICSI) | ||||||
| ICSI not used | 12,487 (33.3) | 15,275 (31.1) | <0.0001 | 11,320 (32.8) | 20,448 (30.7) | <0.0001 |
| ICSI used | 24,963 (66.7) | 33,802 (68.9) | 23,216 (67.2) | 46,050 (69.3) | ||
| Use of assisted hatching | ||||||
| Assisted hatching not used | 30,474 (81.3) | 38,321 (78.0) | <0.0001 | 26,426 (76.4) | 46,274 (69.5) | <0.0001 |
| Assisted hatching used | 7017 (18.7) | 10,802 (22.0) | 8148 (23.6) | 20,281 (30.5) | ||
| Use of preimplantation genetic diagnosis (PGD)c | ||||||
| PGD not used | 29,437 (78.5) | 37,892 (77.1) | <0.0001 | 26,774 (77.4) | 50,737 (76.2) | <0.0001 |
| PGD used | 906 (2.4) | 1157 (2.4) | 1353 (3.9) | 2681 (4.0) | ||
| PGD missing | 7148 (19.1) | 10,074 (20.5) | 6447 (18.7) | 13,137 (19.7) | ||
Data are restricted to fresh, non-donor embryos. Double embryo implantation is when the number of fetal heartbeats on 6-week ultrasound is the same or exceeds the number of embryos transferred, e.g., when double embryo transfer results in two or more heartbeats. Favorable prognosis is defined as those who underwent their first ART cycle and had ≥1 embryo cryopreserved. Less favorable prognosis (not shown) is defined as those who had previous ART cycle(s), no previous live births, and no extra embryos cryopreserved. Average prognosis patients were those who did not meet qualifications for either group. Less favorable prognosis patients were excluded from this analysis
aRace was not reported in many instances
bPercentages do not sum to 100 because patients may have more than one diagnosis
cData not collected in the NASS before 2004
In the adjusted log-binomial models, having ≥1 prior pregnancy and day 5/6 blastocyst ET was positively associated with double embryo implantation following DET irrespective of prognosis group (Table 2). Additionally, in the average prognosis group, diagnosis of ovulatory disorder, male factor infertility, and having >10 oocytes retrieved were positively associated with double embryo implantation in DET. Ages 35 and older, tubal infertility, diminished ovarian reserve, uterine factor infertility, ≥1 prior spontaneous abortion, use of ICSI, and assisted hatching were negatively associated with double embryo implantation following DET in both prognosis groups.
Table 2.
Predictors of double embryo implantation following double embryo transfer, stratified by prognosis, USA, 2000–2012
| Favorable prognosis (n = 36,768) | Average prognosis (n = 34,574) | |||||
|---|---|---|---|---|---|---|
| Rate of double embryo implantation (%) | RR (95 % CI) | aRR (95 % CI) | Rate of double embryo implantation (%) | RR (95 % CI) | aRR (95 % CI) | |
| Maternal age (years) | ||||||
| <35 | 27.6 | Reference | Reference | 18.9 | Reference | Reference |
| 35–37 | 22.2 | 0.80 (0.78–0.82) | 0.79 (0.77–0.81) | 13.8 | 0.73 (0.71–0.75) | 0.76 (0.74–0.78) |
| 38–40 | 16.5 | 0.60 (0.57–0.62) | 0.57 (0.54–0.60) | 7.6 | 0.40 (0.39–0.42) | 0.47 (0.44–0.49) |
| ≥41 | 9.0 | 0.33 (0.27–0.39) | 0.32 (0.27–0.38) | 1.7 | 0.09 (0.08–0.10) | 0.13 (0.11–0.15) |
| Infertility diagnosis | ||||||
| No tubal factor | 26.0 | Reference | Reference | 15.0 | Reference | Reference |
| Tubal factor | 23.7 | 0.91 (0.89–0.93) | 0.89 (0.87–0.92) | 14.2 | 0.95 (0.93–0.97) | 0.93 (0.91–0.96) |
| No endometriosis | 25.6 | Reference | Reference | 14.8 | Reference | Reference |
| Endometriosis | 25.4 | 0.99 (0.97–1.02) | 0.98 (0.95–1.00) | 15.1 | 1.02 (0.99–1.05) | 1.00 (0.97–1.03) |
| No uterine factor | 25.7 | Reference | Reference | 14.9 | Reference | Reference |
| Uterine factor | 23.0 | 0.89 (0.85–0.94) | 0.92 (0.88–0.97) | 12.5 | 0.84 (0.79–0.88) | 0.93 (0.88–0.98) |
| No ovulatory disorder | 25.0 | Reference | Reference | 14.1 | Reference | Reference |
| Ovulatory disorder | 28.0 | 1.12 (1.10–1.15) | 1.02 (0.99–1.04) | 18.9 | 1.34 (1.31–1.38) | 1.08 (1.05–1.11) |
| No diminished ovarian reserve | 25.9 | Reference | Reference | 16.0 | Reference | Reference |
| Diminished ovarian reserve | 20.7 | 0.80 (0.77–0.84) | 0.90 (0.86–0.94) | 7.9 | 0.50 (0.48–0.52) | 0.84 (0.80–0.88) |
| No male factor | 25.7 | Reference | Reference | 14.2 | Reference | Reference |
| Male factor | 25.4 | 0.99 (0.97–1.01) | 1.00 (0.97–1.03) | 15.7 | 1.11 (1.08–1.13) | 1.08 (1.05–1.11) |
| Not unexplained | 25.6 | Reference | Reference | 15.0 | Reference | Reference |
| Unexplained | 25.3 | 0.99 (0.96–1.02) | 0.98 (0.93–1.02) | 13.3 | 0.88 (0.86–0.91) | 0.91 (0.87–0.96) |
| Number of prior pregnancies | ||||||
| 0 | 25.8 | Reference | Reference | 14.7 | Reference | Reference |
| 1 | 25.8 | 1.00 (0.98–1.02) | 1.05 (1.03–1.08) | 15.9 | 1.08 (1.06–1.11) | 1.16 (1.13–1.19) |
| ≥2 | 24.4 | 0.95 (0.93–0.97) | 1.06 (1.02–1.10) | 14.0 | 0.95 (0.93–0.98) | 1.19 (1.14–1.23) |
| Number of prior spontaneous abortions | ||||||
| 0 | 25.9 | Reference | Reference | 15.0 | Reference | Reference |
| 1 | 24.6 | 0.96 (0.93–0.98) | 0.94 (0.91–0.97) | 15.0 | 0.99 (0.97–1.02) | 0.94 (0.91–0.97) |
| ≥2 | 23.8 | 0.92 (0.89–0.95) | 0.91 (0.87–0.96) | 13.1 | 0.87 (0.84–0.90) | 0.88 (0.84–0.92) |
| Stage of transfera | ||||||
| Cleavage | ||||||
| Day 2/3 | 19.2 | Reference | Reference | 10.4 | Reference | Reference |
| Blastocyst | ||||||
| Day 5/6 | 30.2 | 1.57 (1.54–1.61) | 1.58 (1.51–1.65) | 21.0 | 2.03 (1.99–2.07) | 1.67 (1.60–1.75) |
| Number of oocytes retrieved | ||||||
| 1–10 | 22.8 | Reference | Reference | 10.3 | Reference | Reference |
| 11–15 | 25.5 | 1.12 (1.09–1.15) | 1.03 (1.00–1.05) | 17.6 | 1.71 (1.67–1.76) | 1.22 (1.18–1.24) |
| ≥16 | 26.7 | 1.17 (1.14–1.20) | 1.01 (0.98–1.04) | 20.2 | 1.97 (1.92–2.01) | 1.26 (1.22–1.30) |
| ICSI | ||||||
| ICSI not used | 26.7 | Reference | Reference | 15.8 | Reference | Reference |
| ICSI used | 25.0 | 0.94 (0.92–0.96) | 0.89 (0.86–0.91) | 14.4 | 0.91 (0.89–0.93) | 0.84 (0.81–0.87) |
| Use of assisted hatching | ||||||
| Assisted hatching not used | 26.7 | Reference | Reference | 17.3 | Reference | Reference |
| Assisted hatching used | 21.6 | 0.81 (0.79–0.83) | 0.93 (0.88–0.98) | 10.0 | 0.58 (0.56–0.59) | 0.85 (0.81–0.90) |
Data are restricted to fresh, non-donor embryos. The results are of log-binomial regression. The model included maternal age, infertility diagnosis, number of prior pregnancies, number of prior spontaneous abortions, stage of transfer (days 2/3 and 5/6 only), number of oocytes retrieved, ICSI, and assisted hatching. Model was adjusted for year of transfer as a continuous variable
aRR adjusted risk ratio, ICSI intracytoplasmic sperm injection
aCycles with a transfer day that was not day 2/3 or 5/6 were omitted from the model
The strongest positive association with double embryo implantation in DET was the use of day 5/6 blastocyst ET compared with day 2/3 cleavage ET [favorable prognosis: aRR = 1.58 (95 % confidence interval (CI) 1.51–1.65), average prognosis: aRR = 1.67 (95 % CI 1.60–1.75)]. The strongest negative associations were any age category ≥35 compared to age <35 [ages 35–37: favorable aRR = 0.79 (95 % CI 0.77–0.81) and average aRR = 0.76 (95 % CI 0.74–0.78), ages 38–40: favorable aRR = 0.57 (95 % CI 0.54–0.60) and average aRR = 0.47 (95 % CI 0.44–0.49), ages ≥41: favorable aRR = 0.32 (95 % CI 0.27–0.38) and average aRR = 0.13 (95 % CI 0.11–0.15)].
Discussion
In our study, using the national data on fresh, non-donor ART cycles, we found that the rate of single embryo implantation following eSET and double embryo implantation following DET increased; however, rates of single embryo implantation following non-eSET stayed the same while triple embryo implantation following TET and quadruple embryo implantation following QET+ decreased from 2000 to 2012. Among patients with a favorable or average prognosis who had at least one embryo implant, double embryo implantation of two embryos was associated with age less than 35 years as well as day 5/6 blastocyst transfer and having ≥1 prior pregnancy. Additionally, in women with an average prognosis, double embryo implantation was associated with having >10 oocytes retrieved.
Since 2000, rates of single embryo implantation following eSET and double embryo implantation following DET increased. The increase in elective single embryo implantation rate following SET, especially since 2004, coincides with the publication of the first ASRM/SART guidelines promoting eSET [20]. Additionally, Kulkarni et al. showed that the proportion of SET that is eSET has increased over time [2]. The trend observed in our study also follows the previously described increase in eSET frequency since 2004 [10] and concurrent decreases in the number of higher-order embryo transfers [2, 9].
Given the increase in single embryo implantation rates following eSET and double embryo implantation rates following DET, there may be opportunities for the additional use of eSET instead of DET to prevent multiple births. Technological improvements in extended embryo culture and increased practitioner experience with choosing appropriate candidates for eSET are likely the underlying causes of the higher eSET rates and elective single embryo implantation rates following SET and double embryo implantation rates following DET [10, 21]. The decrease in triple embryo implantation rates following TET and quadruple embryo implantation rates following QET+ may also be due to selecting patient candidates for TET and QET+ who are expected to have lower embryo implantation rates. Over time, the proportion of higher-order transfers among all transfers have decreased, while SET and DET have increased [2].
We found that from 2000 to 2012, women with favorable and average prognosis who were younger than 35 had higher rates of double embryo implantation than older women after DET. Our data fits with prior studies of eSET that have also shown younger age to be a predictor of success [10, 11, 22, 23]. The ASRM/SART guidelines on embryo transfer suggest eSET in women who are younger than 35 and have a favorable prognosis; however, these guidelines do not comment on those with an average prognosis [9]. Although the guidelines lay a framework for transfer in “favorable” and “all other” populations, there is still a need to reduce the number of multiple birth pregnancies in the USA [24]. A study of the 2012 NASS fresh and donor data only showed that 93.4 % of all multiple births in 2012 resulted from the appropriate use of the ASRM/SART guidelines [9, 24]. Given that approximately one third of the average prognosis DET resulting in clinical pregnancy results in double embryo implantation, patients and clinicians may consider transferring one embryo instead of two in this population. This is especially important because many women in the average prognosis category may otherwise be counseled towards DET, while pregnancy would be safer with eSET. A prediction model of success in ART by Luke and colleagues show an adjusted odds ratio of 27.25 for a multiple birth pregnancy following DET compared to SET [25].
The strongest predictor of double embryo implantation in DET for both the favorable and average prognosis groups was day 5/6 blastocyst ET compared to day 2/3 cleavage ET, which was consistent with previous studies [12, 24]. Previous studies of implantation following eSET have also shown blastocyst-stage ET to be associated with higher implantation rates than cleavage-stage ET [10, 22, 26]. Although impossible to study, it is certainly possible that if an embryo can survive until blastocyst stage for transfer, then the embryo is inherently of better quality, whereas some embryos that are transferred at cleavage stage may not have survived in culture until blastocyst stage and are of lower quality [21, 27]. Additionally, having >10 oocytes retrieved compared to ≤10 oocytes retrieved with an average prognosis was independently predictive of double embryo implantation, which is also consistent with prior studies [13, 14], but has not been shown to be a predictor of successful eSET. An additional study using the SART CORS database showed higher live birth rate with >10 oocytes retrieved compared to ≤10 oocytes retrieved [28]. Thus, practitioners may consider using the number of oocytes retrieved as another factor to help counsel average prognosis women when choosing DET versus eSET. Other studies found that a greater number of high-quality or supernumerary embryos was associated with double embryo implantation in DET [12, 14, 16–18, 29] and with successful eSET [10, 22, 23]; however, we took this variable into account when stratifying our prognosis groups. A sub-analysis of the DET favorable and DET average prognosis groups was performed to examine what percentage of each group had embryos cryopreserved. The vast majority of the favorable prognosis group had embryos available for cryopreservation, whereas the average prognosis group was more variable, reflecting the inherent way the groups were divided.
The strongest negative predictor of double embryo implantation in both prognosis groups was age ≥35 years compared to age <35 years. Numerous studies also indicate that younger age is a good prognostic indicator for double embryo implantation following DET [12–16]. Despite this association with age, women 35–37 years with a favorable prognosis are more likely to have double embryo implantation following DET than women <35 years with an average prognosis. Clinicians may also consider counseling women with favorable prognosis who are <38 years on having eSET instead of DET to lower the risk of multiple gestation pregnancies.
Our findings have several limitations. First, the NASS does not collect data on embryo morphology or the total number of embryos fertilized. Instead, we used the number of embryos available for cryopreservation, which has been shown to be an acceptable proxy for embryo quality [30, 31]. Additionally, the definitions used to divide prognosis groups differ from the ASRM/SART guidelines on the number of embryos transferred during ART. While the favorable prognosis group is narrow in inclusion, the average prognosis group is quite heterogeneous and our conclusions regarding this group need to be considered in light of the heterogeneity. Additionally, our study was limited to fresh cycles since they are the most common transfer type (66.6 % of cycles in 2011), although the number of frozen cycles is increasing, having tripled from 16,383 cycles in 2002 to 46,779 cycles in 2013 (24.5 % of cycles in 2013) [5]. It will also be important to assess predictors of double embryo implantation in patients with frozen cycles, given that a recent study has shown higher pregnancy rates in frozen transfers compared to fresh, non-donor transfers [32]. The NASS includes only data from ART cycles in the USA; thus, conclusions are only generalizable to cycles performed in the USA. We also cannot account for monozygotic twinning in our study. In instances in which one embryo did not implant, but monozygotic twinning of the remaining implanted embryo occurred, the embryo transfer would appear to have double embryo implantation. Monozygotic twinning is infrequent, however, occurring approximately 0.9 % following ART, compared to the natural rate of approximately 0.4 % [33].
Clinical impression
Although the rate of higher-order embryo transfers in the USA has decreased and the rate of eSET has increased over time, over half of the embryo transfers performed in 2011 were DET [2]. Many of these DETs result in double embryo implantation, leading to multiple gestation pregnancies that may result in maternal and neonatal complications [1, 4–7, 34]. Our study supports wider implementation of single embryo transfer, particularly in women with a favorable or average prognosis who are younger (<35 years) or have blastocyst embryos available for transfer and women with an average prognosis from whom >10 oocytes are retrieved. It is known from prior studies that factors such as younger age and transfer of a blastocyst embryo are associated with successful eSET, and these factors are already incorporated into the ASRM/SART guidelines. We establish that these factors are likewise associated with double embryo implantation following DET. Since many of the factors predictive of successful implantation are already known, physicians may consider using our findings to further tailor patient counseling, especially in patients who are considering DET when they are, in fact, good candidates for eSET. The novel aspect of our study is that we stratified our comparisons by prognosis group, which may be also useful in counseling average prognosis patients on the number of embryos to transfer. Indeed, we found that a large portion of women with an average prognosis have double embryo implantation with DET and would likely be good candidates for eSET. If patients and physicians discuss these factors while making decisions about the number of embryos to transfer, the use of eSET may continue to increase with a corresponding decline in multiple gestation pregnancies and associated maternal and neonatal morbidity and mortality.
Acknowledgments
Caitlin Martin received support from the Marianne Ruby Award in Obstetrics and Gynecology from the Department of Gynecology and Obstetrics of Emory University. We acknowledge Dr. Luca Gianaroli for his help with the initial concept of the study and Dr. Jennifer Kawwass for her critical evaluation of the manuscript.
Abbreviations
- SET
Single embryo transfer
- DET
Double embryo transfer
- TET
Triple embryo transfer
- QET+
Quadruple or higher embryo transfer
- ET
Embryo transfer
- eSET
Elective single embryo transfer
- ART
Assisted reproductive technology
Compliance with ethical standards
Ethical approval
The study was approved by CDC’s institutional review board.
Authors’ role
All authors helped design the study and critically edit the manuscript. C.M. wrote the manuscript. J.C. performed the statistical analysis. C.M., J.C., S.B., and D.K. worked on the data analysis. All authors approved the final version of the manuscript.
Disclaimer
The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.
Footnotes
Capsule
Predictors of double embryo implantation following double embryo transfer are age <35 years and blastocyst transfer in patients with favorable or average prognosis and retrieval of >10 oocytes in patients with average prognosis. Physicians will find this information useful when counseling patients with these characteristics, and these patients may be good candidates for eSET, which would reduce multiple gestations and associated complications.
Contributor Information
Caitlin Martin, Phone: 404-251-8812, Email: Caitlin.Martin@emory.edu.
Jeani Chang, Email: zsj9@cdc.gov.
Sheree Boulet, Email: sbu1@cdc.gov.
Denise J. Jamieson, Email: djj0@cdc.gov
Dmitry Kissin, Email: dtk3@cdc.gov.
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