Placental location and pregnancy outcome

Abstract

Objective

The purpose of this study was to determine if placental location is associated with adverse pregnancy outcome and to assess whether any association exists between different blood groups and location of the placenta.

Material and Methods

Medical records of women were reviewed retrospectively and placental position as documented in the case notes at routine antenatal (20–38 weeks) ultrasonography was identified. Placental position was categorised as anterior, posterior and fundal. Association of placental location with foeto-maternal outcome and different blood groups was noted.

Results

A total 474 case notes of women were analysed for placental location, feto-maternal outcome and blood groups. Anterior placenta was found to have a relation with a greater risk of pregnancy-induced hypertension, gestational diabetes mellitus and placental abruption (p<0.001), while posterior placenta had a significant association with preterm labour (p<0.001). Regarding foetal outcome, an anterior placenta was significantly associated with intrauterine growth retardation and intrauterine foetal death (p<0.001). The majority (54%) of women with an anterior placenta were O-positive blood group, while 46% of women in the posterior placenta group were A-positive blood group (p<0.001).

Conclusion

Anterior placental implantation is associated with an increased risk of pregnancy-induced hypertension, gestational diabetes mellitus, placental abruption, intrauterine growth retardation and intrauterine foetal death. Posterior placenta has a significant association with preterm labour and A-positive blood group. Anterior placenta is common in women with O-positive blood group. Placental location may be an important determinant of pregnancy outcome.

Introduction

In Saudi Arabia, screening ultrasonography (USG) of a large proportion of pregnant women is undertaken and they generally receive at least one obstetric USG for gestational age, amniotic fluid volume, foetal anatomic survey and placental location. Are these implantation sites predictive of an adverse pregnancy outcome? Only a limited number of investigators have dealt with placental implantation site and pregnancy outcome (1–3). Uterine blood supply is not uniformly distributed. The site of implantation and resultant location of the placenta within the uterus are likely important determinants of placental blood flow and therefore pregnancy success (4). There has been extensive research on low placental implantation because of the importance of detecting placenta previa. Only a few studies have been undertaken on other aspects of placental position and possible impact on pregnancy outcome. These studies reported that placental location might have implications for poor pregnancy outcome including preterm birth (5), small for gestational age (SGA) (4), foetal malposition, malpresentation and the development of pre-eclampsia (1, 6). In theory, lateral placental location could contribute a higher risk of foetal intrauterine growth retardation (IUGR). A case-control trial conducted in the USA revealed that women with their placenta located in the fundus carry an increased risk of premature rupture of membranes with all the consequential adverse sequelae (5). Reports on an association between IUGR and placental locations (other than placenta previa) have been conflicting (1, 7, 8).

This study was designed to investigate:

1. The association between placental location and foeto-maternal outcome of pregnancy.

2. The relationship of different maternal blood groups with placental location.

Material and Methods

This retrospective study was conducted at a Abha General Hospital, Abha, KSA. Our study group included women with singleton pregnancy, delivered vaginally after 28 weeks of gestation, who had prior documentation of placental location on the basis of antenatal USG. The study outcome was the relationship of different placental locations with antenatal foeto-maternal complications, like pregnancy-induced hypertension (PIH), gestational diabetes mellitus (GDM), placental abruption, bad obstetric history (BOH), preterm delivery, intrauterine growth retardation (IUGR), intrauterine foetal death (IUFD) and neonatal outcome. The records of 500 subjects were ascertained. It is our departmental policy to offer a routine scan at 18–20 weeks or later if the booking is late. The details of placental locations are recorded apart from other parameters. We categorised each placenta as anterior, posterior and fundal. Placentas occupying the left or right region of the anterior and posterior uterine walls were considered anterior and posterior, respectively. The following data were collected - maternal age, gravidity, parity, number of miscarriages, gestational age at delivery, maternal blood group, birth weight (BW) of the baby, and 1 minute and 5 minute Apgar scores. The data were analysed using SPSS version 17. The results have been expressed in terms of probability (p) value. A chi squared test was used for categorical data and unpaired t test for continuous data. A p-value <0.05 was considered statistically significant. The study was approved by our Institutional Research and Ethics Committee.

Results

The study group consisted of 500 cases who delivered vaginally, during November and December 2012. Of these, 10 cases who delivered before 28 weeks, 10 women who came fully dilated without prior documented placental location by USG and six cases that had multiple pregnancies were excluded, leaving a total of 474 women. Table 1 describes the characteristics of the pregnant women according to placental location. There were no differences between baseline characteristics among three placental location categories. Fundal location was noted in 46%, anterior in 28% and posterior in 26% women (Figure 1). Neonatal outcome according to placental locations are shown in Table 2. There was no significant difference in gestational age at birth, mean BW and Apgar scores. Table 3 shows the association of different maternal blood groups with placental location. The commonest blood groups were O-positive (49%, n=236) and A-positive (36%, n=173). The majority (54%, n=72) of women with O-positive blood group had anterior placenta (p<0.001), while A-positive blood group was associated with posterior placenta in the majority of cases (46%, n=57, p<0.001)

Table 1.

Relationship between placental location and maternal characteristics

Characteristic	Fundal N=218	Anterior N=133	Posterior N=123	p value
Past history
Gravidity (Mean±SD)	5.4±2.2	5.03±1.7	5.5±2.6	0.55
Parity (Mean±SD)	3.8±1.9	3.54±1.6	3.8±2.1	0.54
Miscarriage (Mean±SD)	0.83±1.5	0.63±2.0	0.9±1.0	0.48
Bad obstetric history (%)	2(0.9)	2(1.5)	0	<0.001
Present age & complications
Maternal age (Mean±SD)	32.9±5.1	31.8±5.2	31.9±4.9	2.45
PIH (%)†	7(3.2)	5(3.7)	2(1.6)	<0.001
GDM (%)‡	6(2.7)	8(6.0)	2(1.6)	<0.001
Placental abruption (%)	5(2.3)	5(3.7)	0	<0.001
PTL (%)¥	0	0	3(2.4)	<0.001

SD: standard deviation;

^†PIH -pregnancy induced hypertension;

^‡GDM - gestational diabetes mellitus;

^¥PTL - pre-term labour

Figure 1.

Location of placenta in 474 women

Table 2.

Relationship of foetal outcome with placental location

Variable	Fundal N=218	Anterior N=133	Posterior N=123	p value
Gestational age (Mean±SD)	37.9±1.5	37.2±2	37.8±1.4	2.45
Foetal weight, grams (Mean±SD)	2857±493.4	2808±658.3	2842±503
APGAR score (mean)
1 min	6.2	6.1	6.2	2.48
5 min	8.7	8.4	8.8	3.55
IUGR (%)†	0	4 (3)	0	<0.001
Congenital anomalies (%)	3 (1.4)	0	0	<0.001
IUFD (%)‡	3 (1.4)	2 (1.5)	0	<0.001

SD: standard deviation;

^†IUGR- intrauterine growth retardation;

^‡IUFD- intrauterine foetal death

Table 3.

Association of placental location with blood groups

Blood group N=(474)	Fundal N=218(%)	Anterior N=133(%)	Posterior N=123(%)	p value
O-positive	108 (49.5)	72 (54)	56 (45.5)	<0.001
O-negative	14 (6.4)	4 (3)	0	<0.001
A-positive	70 (32)	46 (34.5)	57 (46.3)	<0.001
A-negative	6 (2.7)	2 (1.5)	0	0.56
B-positive	14 (6.4)	9 (6.7)	10 (8.1)	0.56
B-negative	2 (0.9)	0	0	0.005
AB-positive	0	0	0	0
AB-negative	4 (1.8)	0	0	0.005

Discussion

The blood supply of the uterus is not uniformly distributed and placental location is an important determinant of placental blood flow, as measured by uterine artery Doppler velocimetery (7, 9, 10). There are limited data on the association between placental location, pregnancy complications and perinatal outcome. Some researchers have described that placental location has implications for poor pregnancy outcomes, including preterm birth (5) and small for gestational age (SGA) (4). This study showed a significant association between posterior placenta and preterm labour. In contrast, we did not find any association with SGA. Our finding is consistent with another report (11). Hadley et al. (5) reported that a placenta located in the fundus carries a significantly higher risk of premature rupture of the membrane. They presumed that fundal location of the placenta places the weakest point of the membrane over the cervical os and thus predisposes the women to premature rupture of membrane with all of the consequential adverse sequelae. Contrarily, we found a significant association of posterior placenta with preterm labour. This is probably because placenta located on the posterior uterine wall may be somehow less efficient due to the anatomy of that wall (12). As a result of uneven uterine blood supply (4), the posterior wall of the pregnant uterus is longer (13) and somewhat thicker (14). Each of these factors may affect uterine blood supply, especially as the uterus expands to accommodate the pregnancy. Janewarland et al. (12), in a case-control study, reported that posterior placenta is statistically more likely to result in a still birth. They described that its exact cause is not clear but they put forward three possible hypotheses: the structure of posterior uterine wall is somehow at fault, there may be associated intrauterine factors with the posterior-located placenta, or the pregnant woman’s sleeping position is the problem. However, we did not observe such relation. Rather, this study found a significant correlation between fundal and anterior placenta with IUFD (Table 2). The present study showed a statistically significant association of anterior placenta with an increased incidence of PIH, GDM, BOH, placental abruption, IUGR and IUFD. One of the explanations may be non-uniform uterine blood supply or it might be by chance due to the small sample size; further research is needed to verify these findings. In line with Karthika et al. (15), we did not observe a significant difference in mean BW among different placental location groups. The current study showed that IUGR pregnancies had a significant association with anterior placentas. Although the mean BW of anteriorly placed placental pregnancies is low, it is not significant because the number of IUGR pregnancies is only four out of 133 cases. However, Lucy et al. (4) described a positive correlation between IUGR and lateral placentas, rather than anterior and posterior. Kofinas et al. (7) reported that unilateral placentas are more common than central (anterior and posterior) ones in pregnancies with IUGR and/or pre-eclampsia. Consistent with our findings, Booth et al. (16) reported a significant association between fundal placentation and PIH. The present study revealed a significant association of fundal and anterior placentas with PIH and IUGR. In contrast, a recent population-based case-control study of more than 3000 pregnancies (10) reported that the risk of having a foetus with IUGR was not increased by the site of placental implantation. However, that study grouped placental locations in three broad categories (low, high lateral and high fundal) which did not differentiate central (anterior or posterior) placenta. The observation that most of the placentas in this study were located in fundus is not consistent with prior reports (16–18). At least in theory, a placenta which is primarily implanted near the uterine and/or ovarian arteries might receive more blood than one implanted centrally, whether anterior or posterior, and this could account for poor pregnancy outcome with anterior placenta, as seen in our study. In the three groups of placental location (fundal, anterior and posterior), the mean gestational age and BW were found to be almost the same; however, some have reported the negative impact of fundal location of placenta on these findings. Kalanithi et al. (4) studied the possible influence of placental location on the Apgar scores of newborns. They described the location of the placenta as either fundal, uterine body or lower uterine segment. They found no case of low Apgar score (<4) in the lower uterine segment group, whereas they found that the higher the placenta was situated in the uterus, the greater the incidence of an Apgar score <4 (i.e. 0.6% in the uterine body group and 2.4% in the fundal group). Our study showed no correlation between low Apgar score and placental location. We found no case with Apgar score <4.

We observed that anterior placenta have a strong association with O-positive blood group and posterior placenta with A-positive blood group. Therefore, O-positive blood group women are more prone to antenatal complications, which are commoner in anterior placenta women. To the best of our knowledge, either no work or only small studies have been performed in this regard. Large scale studies are required to confirm these findings. The retrospective nature of this study is a limitation, chiefly because of “observer variation” as a result of different sonographers locating the placental position with variable reporting styles and experience. Nevertheless, a large prospective study, where the placental positions are determined by only one experienced sonographer, would be useful to confirm the findings of this study.

In summary, pregnancies with anterior placenta are complicated by PIH, GDM, placental abruption, IUGR and IUFD as compared to fundal or posterior ones. Anterior placenta is common in O-positive and posterior placenta in A-positive blood group. Women with posterior placenta have a greater risk of premature delivery. This study supports the hypothesis that the location of the placenta is associated with pregnancy success. Therefore, placental location may be an important determinant of pregnancy outcome. Additional research is needed to confirm this observation and to determine whether pregnancies with anterior placenta may benefit from more intensive monitoring.