Study links eldest daughters' early maturation to prenatal stress, lending nuance to 'eldest daughter syndrome'

A UCLA-led study published in the February issue of Psychoneuroendocrinology found that first-born daughters tend to show signs of adrenal puberty earlier when their mothers experience high prenatal stress, a pattern researchers say could help explain emotionally mature caregiving roles for eldest girls in difficult environments.

Researchers tracked families for 15 years, from the pregnancy stage through their children’s adolescence. The study recruited women from two Southern California obstetric clinics during routine first-trimester visits. On average, the women were 30 years old and pregnant with one child; about half were experiencing their first pregnancy. All participants were nonsmokers and not using steroid medications, tobacco, alcohol or other recreational drugs during pregnancy, and all were over 18.

At five points during pregnancy, researchers assessed maternal stress, depression and anxiety, then tracked puberty-related traits in the children after birth. They separately measured indicators of adrenal puberty—such as body hair, skin changes and growth spurts—along with gonadal puberty signs like breast development or the onset of menstruation in girls and voice changes in boys. About 48% of the children were female and 52% male.

The core finding: among first-born daughters, signs of adrenal puberty appeared earlier when their mothers had experienced higher levels of prenatal stress. The researchers concluded that early social maturation could be an adaptive response, enabling an older daughter to help care for younger siblings when resources are limited. Adrenal puberty, they note, does not include the physical changes associated with puberty that enable reproduction, such as breast development or menarche in girls, or testicular enlargement in boys.

One co-author, Jennifer Hahn-Holbrook of the University of California, Merced, explained the evolutionary logic: when times are hard and a mother is stressed during pregnancy, accelerating a daughter’s social maturation can provide a “helper-at-the-nest” sooner, supporting the survival of younger offspring. “It gives mom a helper-at-the-nest sooner, aiding the women in keeping the latter offspring alive in difficult environments,” she said.

The study also found that this association did not hold for boys or for daughters who were not the first-born. Researchers suggested that male children may contribute less to direct childcare than females, reducing any adaptive incentive to accelerate social puberty in sons. Additionally, prior research indicates female puberty timing is more malleable in response to early life experiences than that of males.

The authors emphasized that adrenal puberty is distinct from full puberty, as it does not involve breast development or the onset of menstruation. They stressed that the findings are not a medical diagnosis but rather one piece of evidence about how prenatal stress can shape child development in nuanced ways. The research team described the results as “first-of-its-kind” and noted their significance for understanding fetal programming—the idea that stress and other factors during pregnancy can influence offspring long after birth.

Molly Fox, an UCLA anthropologist and co-author of the study, framed the findings as an evolutionary lens on development. “This is a first-of-its-kind finding and is fascinating to look at through an evolutionary lens,” Fox said in a press release. She noted that the idea of fetal programming helps explain why a fetus might adapt its life trajectory in response to cues about the world its mother expects it to enter.

The study’s release arrived as broader cultural conversations about eldest daughters gained momentum online. Writers and researchers alike have long debated whether eldest daughters bear unique caregiving burdens, with some arguing that the youngest siblings often rely more on their elder sisters for support. For author Y. L. Wolfe, who has described feeling like a “second mom” to her siblings, the findings resonate on a personal level. Wolfe told HuffPost that she did experience true puberty around age 12, though she suspects she also underwent earlier cognitive maturation, a combination that aligns with the study’s emphasis on social and cognitive development in early adolescence.

Beyond its immediate implications, the researchers see potential links to long-term outcomes. Earlier work has suggested eldest daughters may pursue leadership roles and achieve higher levels of success compared with other siblings, though the new study frames those patterns as potentially rooted in early adaptations rather than purely cultural expectations. Still, the team cautions that such maturation is not universal and is tied to specific familial and prenatal stress contexts.

The research contributes to a growing field of inquiry into fetal programming—how the intrauterine environment can shape development well after birth. Fox and colleagues see their work as a starting point for further studies that could clarify how these early adaptations interact with later life experiences, education, and opportunity. In the meantime, the findings offer a nuanced perspective on why some eldest daughters may emerge with heightened social maturity and caregiving instincts, particularly in families facing economic or emotional stress.

As researchers continue to explore these complex dynamics, experts emphasize that understanding does not imply determinism. Parenting practices, social supports, and access to resources all play critical roles in a child’s development, and individual trajectories will vary widely. The study’s authors hope their work will inform future research on family dynamics and child development, contributing to a more precise picture of how prenatal stress can shape offspring in adulthood as well as in childhood.