Prediction 1: Among healthy pregnancies, males will be heavier than females and will have reduced placental reserve capacity

Males were heavier at birth, consistent with findings that males are heavier as early as the first trimester. ^{Reference Bukowski, Smith and Malone54–Reference Kiserud, Piaggio and Carroli56} We found no difference in placental weight between males and females, in contrast to some studies that have found males to have larger placentas. ^{Reference Roseboom, Painter and De Rooij57–Reference Muralimanoharan, Gao, Weintraub, Myatt and Maloyan61} Placental weight, adjusting for birthweight by including it as a covariate, did not differ between males and females. Thus, we found no evidence that male placentas are more efficient or maintain a reduced reserve capacity, defined as lighter placentas for a given birthweight. However, it remains possible that differences in placental structure and function could lead to sex differences in the ability of the placenta to respond to adverse conditions such as reduced nutrient supply. ^{Reference Christians23,Reference Salazar-Petres, Carvalho, Lopez-Tello and Sferruzzi-Perri62}

Prediction 2: There will be a greater sex difference in birthweight among infants that die prenatally or shortly after birth

If females are more responsive to prenatal insults, while males prioritize growth, we expected that female fetuses that died prenatally or shortly after birth would have reduced their growth more than males in an attempt to survive. As a result, the difference in birthweight between females that survived and those that did not would be greater than that among males, leading to a significant sex by survival interaction. This prediction was supported, but only among those born very preterm. While this result might reflect a difference in strategy between the sexes, an alternative explanation is that males and females may have been susceptible to different causes of death, whereby females were more likely to die from chronic impairments and conditions that reduced growth over a longer time period. We were not able to distinguish these alternatives with the available data. However, given that the risks of gestational diabetes, preterm birth, placental abruption and stillbirth are higher in male pregnancies, ^{Reference Verburg, Tucker, Scheil, Erwich, Dekker and Roberts7–Reference Mondal, Galloway, Bailey and Mathews9,Reference Challis, Newnham, Petraglia, Yeganegi and Bocking63,Reference Broere-Brown, Adank and Benschop64} while the risks of preterm preeclampsia may be higher in female pregnancies, ^{Reference Verburg, Tucker, Scheil, Erwich, Dekker and Roberts7,Reference Broere-Brown, Adank and Benschop64} it is plausible that the prevalence of specific types of insults differs between the sexes.

A similar examination of sex-dependent responses to preeclampsia found a greater reduction in fetal growth in males than in females, ^{Reference Reynolds, Roberts, Bodnar, Haggerty, Youk and Catov49} a result opposite to our prediction. In addition to changes in fetal growth, another potential response to prenatal insult such as maternal vascular malperfusion is accelerated placental maturation, a compensatory response to improve placental transport capacity. However, in this population we previously found that the odds of accelerated placental maturation do not differ by sex. ^{Reference Christians and Grynspan65}

Interpreting the results of such human studies is difficult due to the presence of potentially confounding effects, for example, if males and females are subject to different insults. Animal models, allowing manipulation of prenatal conditions, are useful in this respect. In rodents, the fetal growth response to a reduction in food part way through pregnancy did not differ between the sexes, again inconsistent with the view that males prioritize growth more, or have less placental reserve capacity, than females. ^{Reference Christians, Shergill and Albert25} Similarly, surgical models of reduced uterine perfusion pressure reduce fetal weight in both sexes. ^{Reference Siragher and Sferruzzi-Perri66} While maternal inhalation hypoxia sometimes has sex-dependent effects of fetal growth, results from different studies are conflicting. ^{Reference Siragher and Sferruzzi-Perri66}

Prediction 3: Males are at greater risk of fetal or neonatal death

While we did not observe a difference in perinatal mortality between males and females, a meta-analysis of over 30 million births found an increased risk of stillbirth among males. ^{Reference Mondal, Galloway, Bailey and Mathews9} However, this discrepancy is likely due to the lower sample size in the present study; the increased risk estimated in the meta-analysis (∼10%) was the same as we observed. The mechanisms underlying sex differences in the risk of fetal and neonatal death are not known. Some have speculated that higher mortality in males is the result of increased growth rates making fetuses vulnerable to stressors. ^{Reference Eriksson, Kajantie, Osmond, Thornburg and Barker4–Reference Sutherland and Brunwasser6,Reference Mondal, Galloway, Bailey and Mathews9} However, such interpretations focus on deaths in mid- to late-gestation, ignoring the female-biased mortality that occurs early in gestation, when fetal loss may not be recognized, or fetal sex not recorded. ^{Reference Orzack, Stubblefield and Akmaev10} Thus, it may be that compromised males are more likely to survive past early gestation than compromised females, such that male deaths are more likely to be observed, rather than more likely to occur.

Prediction 4: Males will be more susceptible to long-term effects of early-life adversity

We analyzed 7-year assessments using models that included effects of SGA, prematurity and preeclampsia simultaneously. We tested three interactions (between sex and each of SGA, prematurity and preeclampsia) in each of 16 traits, that is, 48 tests, and found only one interaction to be significant. For comparison, we expected approximately 2 out of 48 tests to be significant due to chance given a Type I error rate of 0.05. The one interaction that was significant suggested that girls were more affected by SGA than boys.

Although we found few interactions with sex, the main effects of SGA and prematurity were significant for most of the 7-year traits we examined, with infants born SGA and/or very premature having lower performance, consistent with previous studies of low birthweight and intrauterine growth restriction in this population. ^{Reference Strauss and Dietz43–Reference Pylipow, Spector, Puumala, Boys, Cohen and Georgieff45} In this population, IQ at 7 years was also associated with birthweight within the normal range, although this association was stronger in males when analyzing same-sex siblings. ^{Reference Matte, Bresnahan, Begg and Susser67} In other populations, extremely low birthweight has been associated with reduced cognitive and academic achievement in both sexes, ^{Reference Grunau, Whitfield and Fay68} although cognitive performance was more impacted by extreme prematurity in males. ^{Reference Marlow, Wolke, Bracewell and Samara69} The mechanisms underlying the effects of SGA and prematurity on long-term cognitive outcomes remain active areas of research. A compromised in utero environment could directly affect brain structure and/or increase fetal exposure to glucocorticoid levels as a result of reduced placental 11β-HSD-2 expression, and/or alter fetal exposure to serotonin produced by the placenta. ^{Reference O’Donnell and Meaney70,Reference Brummelte, Mc Glanaghy, Bonnin and Oberlander71} . In contrast to SGA and prematurity, severe preeclampsia was associated with reduced performance in only three tests. Previous work in this population also found a stronger effect of low birthweight than of hypoxia and ischemia, which was defined in part by the presence of preeclampsia. ^{Reference Seidman, Buka, Goldstein, Horton, Rieder and Tsuang44} Other work did find an association between probable hypoxic-ischemic complication and reduced full-scale IQ at 7 years, and no interaction with sex, ^{Reference Goldstein, Seidman and Buka46} whereas in the present study the effect of severe preeclampsia on full-scale IQ was not significant, although performance IQ was reduced. Placental pathology features characteristic of chronic placental hypoxia were associated with reduced verbal IQ in girls only. ^{Reference Anastario, Salafia, Fitzmaurice and Goldstein47}

A negative effect of preeclampsia on intellectual abilities has also been observed in other populations, ^{Reference Pinheiro, Brunetto, Ramos, Bernardi and Goldani72–Reference Morsing and Maršál74} although reductions in intellectual performance were minimal or not significant at older ages. ^{Reference Seidman, Laor, Gale, Stevenson, Mashiach and Danon75–Reference Factor-Litvak, Straka and Cherkerzian79} Few of these studies report sex-dependent effects, although gross-motor scores were more affected in girls, while fine-motor scores were more affected in boys. ^{Reference Ounsted, Moar, Good and Redman80}

Prediction 5: Analyzing the sexes separately will result in spurious sex-specific effects

Analyzing the sexes separately frequently resulted in significant effects of SGA and/or prematurity in one sex but not the other, even though the sex by adversity interaction was not significant when the sexes were analyzed together. We defined these situations as spurious sex-specific effects, and these were observed in 13 out of 16 traits for SGA, 12 out of 16 traits for prematurity, and 2 out of 16 traits for preeclampsia. While the 16 traits we studied are not independent, these results illustrate that analyzing the sexes separately will frequently produce misleading sex-specific effects.

The problem with analyzing the sexes separately is illustrated by the following scenario: if the difference between SGA and non-SGA infants is significantly different from zero in males, but not in females, it does not necessarily follow that the effect size in males is significantly different from the effect size in females; the non-significant effect in females is not evidence that the effect size in females is actually zero. The false sex-specific effects that are observed when the sexes are analyzed separately reflect an inflation of the statistical error rate. When the sexes are analyzed together and the sex by adversity interaction is explicitly tested, there is a single opportunity to commit a type I or type II error. However, when the sexes are analyzed separately, there are two opportunities to commit each type of error. Even if sample sizes are large, the probability of a type II error (accepting the null hypothesis when it is false) may be substantial for a small effect size. As a result of type II errors, there may be a substantial probability of observing a significant effect in one sex but not the other, even if the actual effect size is the same in both sexes.

While a diversity of statistical approaches are used in this field, many studies test the sexes separately, ^{Reference DiPietro and Voegtline3,Reference Sutherland and Brunwasser6} rather than testing interactions. We suggest that some of the reported sex-specific effects may therefore be false positives. Such false positives will obscure real patterns of sex-dependent effects.

Limitations

We restricted our analyses to primiparous White and Black participants to obtain a more homogenous sample, and to avoid potentially confounding effects. However, as a consequence, our results may not generalize to other groups. Similarly, we included only pregnancies ending at 28 weeks of gestation or later, and thus would not have observed differences occurring earlier.

Our definitions of early-life adversity were limited by the availability of data. For instance, SGA was based entirely on birthweight, and thus would have included fetuses that were intrinsically small but healthy; improved definitions of fetal growth restriction involve both estimates of fetal weight and Doppler abnormalities. ^{Reference Melamed, Baschat and Yinon81} Furthermore, the definition of severe preeclampsia has changed since the NCCP study. Briefly, in NCCP data, severe preeclampsia was defined as the presence of one or more features, whereas the current definition includes severe hypertension and one or more features. ⁸² Furthermore, even using the current definition, preeclampsia is heterogeneous, ^{Reference Leavey, Benton, Grynspan, Kingdom, Bainbridge and Cox83} which could explain why fewer long-term effects were observed for this condition. Although we did not detect any patterns to missing data, these could have introduced bias, for example, if more severe complications were more likely to be missing data because of urgency during delivery. Potentially, these limitations could have led to the inclusion or exclusion of a greater number of complications for one sex than the other.