Summary: Using direct measurements of maternal sex hormones from blood samples taken in the first trimester, researchers found that higher maternal estrogen (oestradiol) levels at six to eight weeks of gestation predict a larger newborn head circumference at birth. This prenatal hormonal influence was markedly stronger for male infants than for female infants.
Key Facts
- Direct validation of a long-standing proxy: Prior research often relied on the 2D:4D digit ratio (the relative lengths of the index and ring fingers) as an indirect indicator of prenatal hormone exposure. A higher 2D:4D ratio has been interpreted as reflecting greater prenatal estrogen exposure and has been associated with larger head size. This study confirms that relationship using direct measurements of maternal blood hormone concentrations rather than an anatomical proxy.
- A critical early window: The study identifies a very early developmental window — just 6 to 8 weeks into pregnancy — during which maternal estrogen levels are predictive of the newborn’s head circumference at birth.
- Stronger effect in males: Although the positive association between early maternal estrogen and newborn head size was observed for both sexes, statistical analysis showed the association to be significantly stronger in male neonates than in female neonates.
- Implications for brain size and cognition: Because head circumference is a practical proxy for baseline brain volume and is linked to later cognitive markers, the results support the idea that prenatal estrogen exposure contributes to neonatal cranial size and may influence subsequent neural development.
- An evolutionary trade-off: The authors and commentators highlight an evolutionary paradox: higher prenatal estrogen in males is associated with later-life health costs, including a greater risk of cardiovascular problems and reduced sperm counts. The study reinforces hypotheses suggesting that increases in brain size may have co-evolved with higher prenatal estrogen exposure, producing both cognitive benefits and biological costs.
Source: Swansea University
Overview
A new collaborative study between Swansea University and the Medical University of Lodz examined how maternal hormones in early pregnancy relate to newborn head circumference, a commonly used indicator of brain size at birth. Rather than relying on indirect measures such as digit ratios, the researchers measured maternal estradiol (E) and testosterone (T) directly from blood samples taken during the first trimester.
Published in the journal Early Human Development, the study focused on a cohort of 47 mother–infant pairs. Researchers took blood samples at two time points in early pregnancy (6–8 weeks and 10–11 weeks) and recorded infant head circumference, length, and birthweight at delivery, along with maternal anthropometric data.
Professor John Manning, a digit ratio specialist with Swansea’s Applied Sports, Technology, Exercise and Medicine (A-STEM) research group, noted that the research provides direct evidence that higher maternal estrogen concentrations in the earliest weeks of pregnancy are positively associated with larger neonatal head circumference, and that this association is more pronounced in males.
In the sample, mothers who carried female fetuses had higher early estrogen and testosterone values than mothers carrying male fetuses, but head circumference at birth did not differ significantly by sex in unadjusted comparisons. Statistical modeling identified maternal estradiol at 6–8 weeks (E1) and male sex as positive predictors of head circumference, independent of testosterone measures, later estradiol levels, and maternal characteristics.
Key Questions Answered:
A: The first trimester establishes the foundational architecture for the developing brain. This study shows that maternal estrogen levels measured at six to eight weeks are positively associated with newborn head circumference, which is strongly linked to brain volume at birth and correlates with later cognitive development. In short, the hormonal environment in early pregnancy helps shape the structural potential for a child’s future brain size and cognitive trajectory.
A: Male and female fetuses develop under different hormonal regimes. Female development tends to occur in a relatively estrogen-dominant context, while male development involves a more complex interplay between endogenous testosterone and maternal hormones. The study found that male cranial growth is especially sensitive to increases in early maternal estrogen, producing a stronger association between early estradiol exposure and head circumference in boys than in girls.
A: The “estrogenized ape” hypothesis proposes that elevated prenatal estrogen levels contributed to human brain expansion over evolutionary time. The researchers and commentators highlight a potential trade-off: higher prenatal estrogen in males has been linked to later-life health costs, including cardiovascular risks and reduced sperm counts. One interpretation is that evolutionary increases in brain size may have come at the cost of male health and fertility, meaning cognitive advantages were accompanied by biological penalties in other domains.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full by editorial staff.
- Additional context was provided by editorial staff to clarify implications and limitations.
About this IQ and developmental neuroscience research news
Author: Kathy Thomas
Source: Swansea University
Contact: Kathy Thomas – Swansea University
Image: The image is credited to Neuroscience News
Original Research: Open access. “First trimester maternal sex steroids and head circumference in newborns” by Anna Kasielska-Trojan, Bogusław Antoszewski, John T. Manning, Marek Kałuża. Early Human Development. DOI:10.1016/j.earlhumdev.2026.106604
Abstract
First trimester maternal sex steroids and head circumference in newborns
Objectives
Previous indirect evidence, including studies using the 2D:4D digit ratio, suggests a positive relationship between prenatal estrogen exposure and neonatal head circumference, with stronger effects in males. This study tests that hypothesis directly by examining maternal first-trimester estradiol (E) and testosterone (T) levels and their association with newborn head circumference.
Material and methods
Estradiol and testosterone were measured from maternal blood at 6–8 weeks (E1, T1) and again at 10–11 weeks (E2, T2). Neonatal head circumference, length, and weight were recorded at birth, alongside maternal anthropometric measures.
Results
The sample included 47 neonates (24 boys) and their mothers. Mothers carrying female fetuses had higher mean values of E1, T1 and E2 than mothers carrying male fetuses. No significant sex differences were found in maternal age, height, weight, BMI, or pregnancy weight gain, nor in neonatal head circumference, length, or birthweight on univariate comparisons. Head circumference correlated negatively with maternal age at pregnancy and positively with E1. Multiple regression identified positive relationships between head circumference and E1 and male sex, with no independent effects for T1, E2, T2, or maternal anthropometrics. Analyses split by sex showed that positive correlations between E1 and head circumference were present for both sexes but were stronger in males.
Conclusion
Neonatal head circumference was positively associated with maternal estradiol measured at 6–8 weeks, independent of maternal testosterone, later estradiol levels, and maternal body measures. The relationship was stronger in male neonates than in female neonates. These direct findings support previous indirect reports linking prenatal estrogen exposure and head circumference and reinforce evidence that these effects are more pronounced in boys than in girls.