Previous studies in singleton pregnancies have found fetal male sex (compared with female fetuses) to be an independent risk factor for adverse pregnancy outcome1,2 including prematurity,2–5 macrosomia,6 cord complications,6 cesarean delivery,6–10 and neonatal morbidity and mortality.11 The mechanisms underlying these observations as well as whether these differences reflect the result of a male-offending factor or a female-protective factor remain unclear. Nevertheless, the finding that androgens have a positive effect on fetal growth12,13 and an inhibitory effect on lung development14,15 led to the hypothesis that these sex-related differences may reflect a hormonal-mediated intrauterine male disadvantage compared with female neonates.
There is evidence from animal studies that androgens are transported between fetuses across fetal membranes and that female fetuses adjacent to a male co-twin have higher levels of testosterone than those adjacent to a female co-twin.16,17 Assuming that this holds true for human pregnancies as well, then unlike-sex twin pregnancies appear to be an appealing natural experiment to assess the hypothesis described above regarding a hormonal-mediated male disadvantage by comparing the outcome of female fetuses having either a male or a female co-twin.
Although in some previous studies, it has been attempted to assess the association between fetal sex and pregnancy outcome in twins, results remain equivocal.18–23 One major limitation of some of these studies is the lack of distinction between dichorionic and monochorionic twins,19,20,23 resulting in an overestimation of adverse outcome in same-sex twin pregnancies.21,22 Lack of adjustment for confounding factors18,19,21,22 and lack of distinction between spontaneous and iatrogenic prematurity18,21,22 may also explain the discrepancy between these studies.
The aim of the current study was to estimate the association between fetal sex and pregnancy outcome in dichorionic twin pregnancies and the effect of male and female fetuses on their opposite-sex co-twin.
MATERIALS AND METHODS
We conducted a retrospective study of all dichorionic twin pregnancies at the Rabin Medical Center in Israel, a university-affiliated tertiary hospital, between January 1995 and December 2006. Exclusion criteria were as follows: gestational age at delivery less than 24 weeks of gestation, birth weight less than 500 g for any of the twins, monochorionic twins, stillbirth or fetal reduction of one of the twins, and major fetal congenital anomalies. The study protocol was approved by the Rabin Medical Center Institutional Review Board.
Maternal demographic and obstetric characteristics, chorionicity (determined by first or early second-trimester ultrasound examination), pregnancy outcome, and perinatal outcome were recorded from the perinatal database and maternal and neonatal medical charts. Gestational age at delivery was based on last menstrual period and, whenever available, confirmation by first-trimester ultrasound examination.
Pregnancies were classified into three groups according to fetal sex (ie, female–female, male–female, and male–male), and pregnancy outcome was compared for the three groups. Neonatal outcome of female neonates from female–female pregnancies was compared with that of female neonates from male–female pregnancies. Similarly, the outcome of male neonates from male–male pregnancies was compared with that of male neonates from male–female pregnancies.
Data analysis was performed with the SPSS 15.0 software (SPSS, Inc., Chicago, IL). The Student t test and one-way analysis of variance were used to compare continuous variables between the groups, and the χ2 test was used for categorical variables. Multivariable logistic regression analysis was used to adjust the risk of prematurity and adverse neonatal outcome for potential confounders. Survival analysis using the Cox proportional hazards model was used to compare the length of gestation for the three groups of twin pregnancies, while adjusting for potential confounding variables. Differences were considered significant when the P value was less than .05. When multiple comparisons were performed, Bonferroni corrections were used as required to maintain an overall type I error rate of 0.05.
Of a total of 4,630 twin pregnancies during the study period, 2,704 (58.4%) were eligible for the study (pregnancies excluded were 1,789 cases of monochorionicity or undetermined chorionicity, 57 cases of fetal reduction or stillbirth of one of the twins, 53 cases of major congenital anomalies, and 27 cases of delivery at less than 24 gestational weeks or birth weight less than 500 g for at least one of the twins). Of the neonates found eligible for the study, 436 (16.1%) were female–female, 1,878 (69.5%) were male–female, and 390 (14.4%) were male–male pregnancies. The demographic and obstetric characteristics of the women in the three twin-pregnancies groups were similar (Table 1).
The mean gestational age at delivery was significantly lower for male–male pregnancies compared with female–female and male–female pregnancies (Table 2). Even after adjustment for potential confounding factors by Cox regression analysis, duration of pregnancy was shortest in the male–male group (hazard ratio 1.3, 95% CI 1.1–1.6) and intermediate in the male–female group (hazard ratio 1.2, 95% CI 1.1–1.4) using the female–female group as a reference (Fig. 1).
The differences in the adjusted risk for preterm delivery was significant for preterm delivery before 31 and 28 weeks of gestation and was related to a higher rate of spontaneous preterm delivery (Table 2). To estimate the absolute increase in the risk for preterm delivery, we calculated the number needed to harm (NNH), which indicates how many male–female or male–male pregnancies (compared with female–female pregnancies) would result in one additional preterm delivery. The NNH for delivery before 31 weeks of gestation was 50 for male–female and 27 for male–male pregnancies. For delivery before 28 weeks, the NNH was 71 and 43, respectively. The risk of less extreme prematurity (less than 34 weeks of gestation) was similar for the three twin-pregnancies groups (Table 2).
Overall, birth weight was significantly higher for male compared with female neonates (Table 3). Male neonates in male–male twin pairs had a lower mean birth weight, a higher rate of fetal growth restriction, and lower rate of discordancy when compared with male neonates in male–female pairs (Table 3).
To determine whether the lower mean birth weight in males from male–male twin pairs was the result of the higher rate of prematurity in this group, we plotted the mean birth weight at each gestational week for males from male–male and male–female twin pairs (Fig. 2A). Starting from 32 weeks of gestation, males from male–female twin pairs had a higher growth rate compared with males from male–male twin pairs, and the difference in the growth rate increased gradually with increasing gestational age (Fig. 2A). For female neonates, the mean birth weight and growth rate were similar, irrespective of whether they developed in the company of a male or female co-twin (Table 3 and Fig. 2B). Table 4 presents the risk of adverse neonatal outcome for male and female neonates from unlike-sex and like-sex twin pregnancies, adjusted for potential confounders.
Female neonates from male–female pregnancies were at increased risk of respiratory distress syndrome (RDS) (NNH=35), intraventricular hemorrhage (IVH) (NNH=42), and convulsions (NNH=77) compared with female neonates from female–female twin pregnancies (Table 4). The rate of RDS and IVH in female neonates from male–female pairs was similar to the overall rate of RDS and IVH observed in male neonates (P=.2 and P=.7, respectively, Table 4). Of the 24 cases of convulsions in this group (female neonates from male–female pairs, Table 4), 13 were attributed to IVH (of them, six cases were of third to fourth degree). Female neonates from female–female pregnancies had a lower overall rate of IVH, but the rate of third to fourth degree IVH was similar to that of female neonates from male–female pregnancies. None of the cases of IVH in this group (female neonatess from female–female pairs) was associated with neonatal convulsions.
Male neonates from male–male pregnancies had a similar rate of neonatal complications compared with male neonates from unlike-sex twin pregnancies except for a higher rate of convulsions (NNH=47) (Table 4). Of the 24 cases of convulsions in male neonates from the male–male group (Table 4), 12 were attributed to IVH (of them, eight cases were of third to fourth degree). For comparison, of the 18 cases of convulsions in male neonates from unlike-sex pregnancies, nine were attributed to IVH (of them, two cases were of third to fourth degree). Thus, although the rate of IVH (overall and third to fourth degree) was similar in male neonates from male–male and male–female twin pairs (Table 4), IVH was more likely to be associated with convulsion in male neonates from male–male compared with male–female pregnancies.
In the current study, we estimated the effect of male and female fetuses on their opposite-sex co-twin in dichorionic twin pregnancies. Our main findings were 1) male–male twin pregnancies, and to a lesser extent male–female pregnancies, were at increased risk of prematurity compared with female–female pregnancies, 2) for male neonates, the presence of a female rather than a male co-twin was associated with a higher birth weight due to both longer duration of pregnancy and a higher in utero growth rate, and 3) for female neonates, the presence of a male rather than a female co-twin was associated with not only an increased risk for prematurity, but also with an increased risk for prematurity-related morbidity.
In singleton pregnancies, the risk of prematurity has been reported to be higher in the presence of a male compared with a female fetus.24 We have recently corroborated these findings in a retrospective study which included over 66,000 singleton pregnancies (Melamed N, Yogev Y, Glezerman M. Fetal gender and pregnancy outcome. J Matern Fetal Neonatal Med. In press). In the current study, we have found this to be true also for twin pregnancies: the risk of prematurity was highest in male–male pregnancies and lowest in female–female pregnancies. Most previous studies in twin pregnancies also found that male–male pregnancies have the highest rate of preterm delivery.18,20–22,25,26 Loos et al18 found that the mean duration of gestation of male–female pairs (36.8±2.7 weeks) was similar to that of female–female pairs (36.9±2.6 weeks), but both study groups had a significantly longer gestation than male–male pairs (36.4±2.8 weeks). They concluded that it is the female who governs the length of gestation. Since the mean gestational age for the three groups in our study followed a similar pattern, this interpretation concerning a female-protective factor that prolongs gestation may also be applied to our results. However, the finding that the risk of preterm delivery at 31 and 28 weeks of gestation in male–female pairs was higher than in female–female pairs and lower than in male–male pairs can also be explained by a potential “dose-dependent” male-offending factor that increases the risk of prematurity. The higher testosterone level in male pregnancies, which has been implicated in the onset of preterm labor,27 may be an example for such a male-related factor.28,29
We have found that, for a male twin, the presence of a female rather than a male co-twin was associated with a higher birth weight. This finding is in agreement with previous reports and had been attributed, as in our study, to both a longer gestation18,21,22 and a higher growth rate18,21 in the presence of a female co-twin. The reason for this observation is unclear. It has been speculated that due to the inherently slower growth of female compared with male fetuses, the male fetuses may be more successful in their competition for nutrients in the presence of a female rather than a male co-twin.30
Androgens have been shown to exert a positive effect on fetal growth.12,13 In addition, there is evidence from animal-model studies that androgens can be transported between fetuses across fetal membranes and that female fetuses adjacent to a male co-twin have higher levels of testosterone than those adjacent to a female co-twin.16,17 Based on these observations, it has been hypothesized that female fetuses would grow faster in the presence of a male compared with a female co-twin.25 However, in the current study, we did not find differences in birth weight and intrauterine growth rate of female fetuses in the presence of a male or a female co-twin, and results from previous studies testing this hypothesis are conflicting.25,31,32,18,22,33
Preterm male neonates in singleton pregnancies have been shown to be at increased risk of respiratory and neurologic morbidity when compared with female neonates of comparable gestational age.11 Considering the possible inhibitory effect of androgens on lung development,14,15 it has been suggested that this male disadvantage may be related in part to the higher level of androgens in male pregnancies.23 Based on this assumption, we aimed to determine whether female neonates having a male compared with a female co-twin are at increased risk of neonatal morbidity. We have found that for female neonates, the presence of a male rather than a female co-twin is associated with an increased risk of respiratory and neurologic morbidity to a level similar to that observed in male neonates.
Recently, Shinwell et al23 analyzed the outcome of very low birth weight neonates from twin pregnancies. In concordance with our results, they found that the adjusted risk of respiratory morbidity in female neonates from male–female pregnancies was similar to that of male neonates and significantly higher than that of female neonates from female–female pregnancies. The authors concluded that the difference in morbidity between male and female preterm neonates represents a male disadvantage as opposed to a female advantage and that this disadvantage may be transferred from male to females in unlike-sex twin pairs, perhaps via an intrauterine paracrine effect.23
We have found a higher rate of short-term neurologic morbidity among male and female neonates having a male co-twin. Interestingly, the rate of IVH for female neonates from male–female pregnancies was very similar to the rate of IVH in male neonates and significantly higher than that observed for female neonates from female–female pregnancies, which supports a male-related offending factor that is associated with an increased risk for IVH. The reason for these observations is not clear. In singleton pregnancies, the higher rate of IVH (overall and third and fourth degree) in male neonates34 has been attributed to the higher susceptibility of male neonates to brain injury due to differences in the steroid hormone environment35,36 and a higher cerebral blood flow.37 Furthermore, the recent findings that the beneficial effects of indomethacin in prevention of IVH and related morbidity is limited to male (versus female) preterm neonates38 provide additional support to a male-related factor in the pathogenesis of IVH and its subsequent brain injury in male neonates. It remains to be investigated whether such a male-related factor may increase the rate of IVH in a female co-twin to reach a magnitude similar to that observed in male neonates.
The main limitation of the current study is its retrospective design. Data, such as maternal body mass index, which may affect birth weight, were not available. Similarly, a large number of pregnancies were excluded from the study because of lack of data regarding chorionicity. Although the main strength of this study is the large group of pregnant women who have been treated at the same institution and according to the same clinical guidelines, this also limits the generalizability of the findings of the current study.
In summary, our results indicate that male and female fetuses affect their opposite-sex co-twins, and its essence is that it is better for a twin (male or female) to share the womb with a female rather than with a male co-twin. Although with regard to prematurity, it is not clear whether the differences are the result of a male- or a female-related factor, the analysis of neonatal outcome for preterm twin neonates supports a male-offending rather than a female-protective factor. In addition to its short-term obstetric and neonatal implications, such an in utero effect of fetuses on their opposite-sex co-twins has also been suggested to carry long-term physical,39,40 cognitive,22 and behavioral41–43 consequences, as well to increase the risk of cancer in adult life.21,44–47
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