Table 1.
Demographic and clinical characteristics of vitrified embryo transfer cycles.
| Variable | All cycles | Group 1 | Group 2 | Group 3 | P-value |
|---|---|---|---|---|---|
| 0–2 years | 2–5 years | >5 years | |||
| Number of thawing cycles, n | 36 665 | 31 565 | 4458 | 642 | |
| Number of frozen embryos transferred cycles, n (%) | 36 432 (99.36%) | 31 386 (99.43%) | 4413 (98.99%) | 633 (98.60%) | |
| Maternal age at oocyte retrieval (years) | 32.31 ± 4.91 | 32.67 ± 4.95 | 30.13 ± 3.97 | 29.38 ± 3.62 | <0.001 |
| <35 | 26 432 (72.55%) | 21 933 (69.88%) | 3912 (88.65%) | 587 (92.73%) | <0.001 |
| 35–37 | 5022 (13.78%) | 4627 (14.74%) | 359 (8.14%) | 36 (5.69%) | |
| ≥38 | 4978 (13.66%) | 4826 (15.38%) | 142 (3.22%) | 10 (1.58%) | |
| Maternal age at FET (years) | 33.16 ± 4.85 | 33.08 ± 4.97 | 33.36 ± 3.99 | 35.41 ± 3.69 | <0.001 |
| Storage time by vitrification (years) | 0.85 ± 1.24 | 0.41 ± 0.39 | 3.23 ± 0.78 | 6.03 ± 0.91 | <0.001 |
| Maternal BMI (kg/m2) | 22.13 ± 3.18 | 22.19 ± 3.20 | 21.78 ± 2.99 | 21.69 ± 3.02 | <0.001 |
| Type of infertility, n (%) | <0.001 | ||||
| Primary | 18 363 (50.40%) | 15 470 (49.29%) | 2538 (57.51%) | 355 (56.08%) | |
| Secondary | 18 069 (49.60%) | 15 916 (50.71%) | 1875 (42.49%) | 278 (43.92%) | |
| Main causes of infertility, n (%) | <0.001 | ||||
| Female factor | 19 057 (52.31%) | 16 046 (51.12%) | 2585 (58.58%) | 426 (67.30%) | |
| Male factor | 6204 (17.03%) | 5434 (17.31%) | 681 (15.43%) | 89 (14.06%) | |
| Male and female factors | 9258 (25.41%) | 8113 (25.85%) | 1034 (23.43%) | 111 (17.54%) | |
| Unexplained | 1913 (5.25%) | 1793 (5.71%) | 113 (2.56%) | 7 (1.11%) | |
| Number of oocytes retrieved | 15.15 ± 8.55 | 14.80 ± 8.60 | 17.14 ± 7.80 | 18.49 ± 8.04 | <0.001 |
| Fertilization method, n (%) | <0.001 | ||||
| IVF | 28 859 (79.21%) | 24 753 (78.87%) | 3575 (81.01%) | 531 (83.89%) | |
| ICSI | 6486 (17.80%) | 5668 (18.06%) | 719 (16.29%) | 99 (15.64%) | |
| IVF + ICSI | 1087 (2.98%) | 965 (3.07%) | 119 (2.70%) | 3 (0.47%) | |
| Number of embryos transferred, n (%) | 0.185 | ||||
| 1 | 18 620 (51.11%) | 16 101 (51.30%) | 2206 (49.99%) | 313 (49.45%) | |
| ≥2 | 17 812 (48.89%) | 15 285 (48.70%) | 2207 (50.01%) | 320 (50.55%) | |
| Development stage of embryo transferred, n (%) | <0.001 | ||||
| Cleavage embryo | 10 630 (29.18%) | 9543 (30.41%) | 948 (21.48%) | 139 (21.96%) | |
| Extended culture of cleavage embryos | 2388 (6.55%) | 1591 (5.07%) | 617 (13.98%) | 180 (28.44%) | |
| Blastocyst | 23 414 (64.27%) | 20 252 (64.53%) | 2848 (64.54%) | 314 (49.61%) | |
| Good-quality embryos transferred, n (%) | <0.001 | ||||
| 0 | 9408 (25.82%) | 7982 (25.43%) | 1226 (27.78%) | 200 (31.60%) | |
| 1 | 19 115 (52.47%) | 16 679 (53.14%) | 2159 (48.92%) | 277 (43.76%) | |
| 2 | 7909 (21.71%) | 6725 (21.43%) | 1028 (23.29%) | 156 (24.64%) | |
| Number of previous embryo transferred, n (%) | <0.001 | ||||
| 0 | 23 160 (63.57%) | 21 834 (69.57%) | 1171 (26.54%) | 155 (24.49%) | |
| 1 | 9030 (24.79%) | 7039 (22.43%) | 1756 (39.79%) | 235 (37.12%) | |
| ≥2 | 4242 (11.64%) | 2513 (8.01%) | 1486 (33.67%) | 243 (38.39%) | |
| Delivery achieved during this IVF/ICSI cycle | <0.001 | ||||
| Yes | 2366 (6.49%) | 273 (0.87%) | 1733 (39.27%) | 360 (56.87%) | |
| No | 34 066 (93.51%) | 31 113 (99.13%) | 2680 (60.73%) | 273 (43.13%) | |
| Endometrial preparation method, n (%) | <0.001 | ||||
| Natural | 11 038 (30.30%) | 9254 (29.48%) | 1568 (35.53%) | 216 (34.12%) | |
| Artificial | 22 475 (61.69%) | 19 538 (62.25%) | 2563 (58.08%) | 374 (59.08%) | |
| Others | 2919 (8.01%) | 2594 (8.26%) | 282 (6.39%) | 43 (6.79%) | |
| Thickness of the endometrium (mm) | 8.84 ± 1.60 | 8.81 ± 1.59 | 9.02 ± 1.68 | 8.80 ± 1.52 | <0.001 |
| Survival rate (%) | 58 300/59 406 (98.14%) | 49 112 (98.15%) | 7859/8012 (98.09%) | 1329/1358 (97.86) | 0.698 |
| Implantation rate (%) | 20 366/54 244 (37.55%) | 17 441/46 671 (37.37%) | 2584/6620 (39.03%) | 341/953 (35.78) | 0.017 |
| Biochemical pregnancy, n (%) | 1342 (3.68%) | 1154 (3.68%) | 164 (3.72%) | 24 (3.79%) | 0.981 |
| Live birth, n (%) | 13 651 (37.47%) | 11 705 (37.29%) | 1725 (39.09%) | 221 (34.91%) | 0.028 |
| Multiple pregnancy, n (%) | 3329 (19.23%) | 2802 (18.84%) | 464 (21.54%) | 63 (22.11%) | 0.005 |
| Ectopic pregnancy, n (%) | 253 (1.46%) | 213 (1.43%) | 36 (1.67%) | 4 (1.40%) | 0.695 |
| Miscarriage, n (%) | 3235 (18.68%) | 2801 (18.83%) | 377 (17.50%) | 57 (20.00%) | 0.285 |
FET, frozen embryo transfer.