Stress During Pregnancy Can Lead to Early Puberty in Firstborn Daughters

A research team at the University of California, Los Angeles (UCLA) has found evidence in a comprehensive long-term study that psychological stress during pregnancy may be linked to changes in children’s early hormonal development. Particularly striking was a pattern observed exclusively in firstborn daughters. This association could not be demonstrated in boys or in children born later.

The study falls within the field of research known as fetal programming. This scientific approach posits that environmental conditions during pregnancy can have long-term effects on a child’s development. This involves not only obvious influences such as nutrition or pollutants, but increasingly also psychological factors such as chronic stress, anxiety, or depressive distress in the mother.

Early Hormonal Activation as a Developmental Phase

The study focuses on so-called adrenal puberty, an early phase of hormonal maturation triggered by adrenal gland activity. During this phase, the production of certain androgens increases—hormones involved in the development of body hair, skin changes, and subtle growth processes. Early changes in emotional and cognitive maturation are also associated with this phase.

It is important to note that this phase should not be equated with puberty itself. Rather, it represents a preliminary stage that precedes full sexual development. In girls, this explicitly does not signify the onset of breast development or menstruation, and in boys, it does not signify the full maturation of the reproductive organs. Instead, it involves early hormonal activation that may provide clues as to how the endocrine system is prepared for later developmental processes. In developmental biology, this phase is often considered sensitive because it reacts relatively strongly to external influences and may influence long-term “settings” in the hormonal system.

Possible Biological Mechanisms Behind Prenatal Stress

Researchers discuss several possible mechanisms through which stress during pregnancy could affect the unborn child. Central to this is the so-called hypothalamic-pituitary-adrenal axis, also known as the body’s stress axis. During prolonged stress, the release of cortisol—a hormone that influences many physiological processes—increases.

Some of this cortisol can—at least indirectly—cross the placenta or alter its protective mechanisms. This can change the fetal hormonal environment, which in turn could influence the development of the child’s own stress regulation. In research, this process is often described as “biological programming.” This does not imply a fixed determination, but rather a shift in developmental probabilities.

Of particular interest is the question of why this effect occurred in the study only in firstborn daughters. One possible explanation is that the first pregnancy is associated with a kind of “biological initial adaptation” of the mother’s body. The mother’s hormonal and immunological systems change significantly during the course of the first pregnancy, which could influence subsequent pregnancies. This could also alter the conditions for the fetus.

Study with Long-Term Observation and Detailed Data Collection

The study was conducted as a prospective long-term study in which pregnant women were followed over several years. Participants were recruited at two maternity hospitals in Southern California and interviewed repeatedly throughout their pregnancies. Their mental health was systematically assessed, with a particular focus on symptoms of stress, anxiety, and depression. These repeated measurements made it possible to analyze not just snapshots but the progression of stress levels throughout the entire pregnancy. After birth, the children were monitored for many years. The researchers examined both physical developmental characteristics and hormonal markers to detect early signs of adrenal maturation. Additionally, saliva samples were analyzed to measure hormone levels that serve as indicators of adrenal gland activity.

An important aspect of the study was controlling for potential confounding factors. These included social and family stressors such as parental separation, economic insecurity, or the loss of a parent. These factors are already known to influence pubertal development and were statistically accounted for to better isolate the specific influence of prenatal stress.

“Firstborn Daughter Syndrome” in a Social Context

The study’s findings have also been widely discussed outside the scientific community, particularly in connection with what is known as “firstborn daughter syndrome.” This term does not originate from medicine but from social media and online communities. There, it is used to describe a pattern in which firstborn daughters often take on responsibilities within the family at an early age. In these discussions, many affected individuals report that they felt responsible for the well-being of younger siblings from a young age or took on household tasks that went beyond the usual scope. This phenomenon is often interpreted as a combination of cultural expectations, family structures, and individual personality.

However, the UCLA researchers explicitly emphasize that their findings do not provide direct evidence for this social theory. Nevertheless, the study has sparked widespread public interest because some have interpreted it as a possible partial biological explanation for a culturally observed pattern. Scientifically, however, this connection remains speculative. It is currently unclear whether hormonal developmental differences actually influence social roles or whether both phenomena arise independently of one another and merely occur in parallel by chance. The researchers view this more as an open area of research than as a confirmed cause-and-effect relationship.

Implications for Developmental Research and Prevention

The study’s findings contribute to a growing understanding of how strongly early environmental conditions can influence human development. Particularly important is the realization that it is not merely the general presence of stress during pregnancy that plays a role, but possibly also its temporal course as well as the specific developmental stage of the fetus during which the stress occurs. Different organ systems and hormonal axes develop at different times, meaning that certain phases may be particularly sensitive to external influences. In developmental biology, this concept is often described as “critical windows.” It assumes that there are periods during which certain systems behave in a particularly plastic manner and can therefore be more strongly shaped by environmental factors. The study provides evidence that the development of the adrenal axis may also fall within such sensitive time windows. This could explain why not all children react equally to prenatal stress.

At the same time, the researchers emphasize very clearly that the results should not be understood as deterministic predictions. These are statistical correlations observed at the group level, not individual fate predictions. Many children born during a psychologically stressful pregnancy develop completely normally and show neither early hormonal changes nor later health abnormalities. Biological development therefore remains highly variable despite prenatal influences and is shaped by many other factors. It is also important to note that prenatal stress is just one of many possible factors influencing child development. Genetic predispositions, the postnatal environment, nutrition, social stability, and later life experiences play an equally significant role. The study cannot therefore be viewed in isolation but rather complements a complex interplay of various biological and social factors that collectively shape a child’s development.

Despite these limitations, the findings could have important long-term implications for prenatal healthcare. They underscore the importance of mental health during pregnancy not only in terms of the mother’s immediate well-being but also with regard to potential long-term effects on child development. This applies in particular to access to psychological care, stress reduction in daily life, and social support systems during pregnancy. In a further step, research could help identify risk factors earlier and develop more targeted support services. However, this is not about pathologizing pregnancies or causing additional anxiety for expectant parents, but rather about gaining a better understanding of biological developmental processes. The goal would be to design preventive measures in such a way that they positively support both the mother’s mental health and the child’s long-term development.

In the long term, the study thus opens up a broader field of research that addresses the intersection of biology, psychology, and the social sciences. It shows that early developmental processes are not exclusively genetically determined but are in a dynamic interplay with the environment. At the same time, the central message remains that human development is robust, flexible, and shaped by many different influences, so that individual risk factors must never be understood in isolation as the cause.

Open Questions and Future Research

Despite the extensive data set, key questions remain unanswered. Particularly striking is the finding that the association between prenatal stress and early adrenal maturation was observed exclusively in firstborn daughters. Why this effect is not seen in boys or in children born later has not yet been conclusively explained and represents one of the most important open questions of the study. One possible explanation could lie in complex interactions between biological development, placental function, and maternal adaptation to pregnancy. It is known that physiological conditions during a first pregnancy can differ from those in subsequent pregnancies. These include changes in the immune system, in hormonal regulation, and in the placenta’s permeability to certain stress hormones.

These factors could theoretically influence how strongly a fetus reacts to maternal stress signals. At the same time, it is unclear whether there are sex-specific differences in the sensitivity of fetal hormonal axes. Previous research suggests that male and female fetuses may react differently to environmental stress, both in terms of growth and later hormonal regulation. However, whether this can explain the effect observed in the study remains purely hypothetical. There are also significant gaps in knowledge at the molecular level. While it is known that stress hormones such as cortisol can influence the development of the hypothalamic-pituitary-adrenal axis, the exact signaling pathways leading to long-term changes in adrenal activity are not yet fully understood. In particular, it is unclear which epigenetic mechanisms may be involved—that is, which changes in gene regulation are triggered by prenatal stress and how permanent these changes are.

Future research will therefore seek to investigate these relationships on multiple levels simultaneously. On the one hand, larger and more ethnically diverse cohorts are needed to determine whether the observed effects are stable and reproducible or whether they may be influenced by specific social or cultural contexts. On the other hand, experimental and molecular biological approaches should help to better understand the underlying mechanisms. A particular focus is on the question of how stress signals during pregnancy are “translated” into long-term biological changes. In this context, it is likely that not only classical hormonal systems play a role, but also epigenetic marks that can alter the activity of certain genes over the long term. Such mechanisms could explain why early environmental conditions remain visible in development even years or decades later. In the long term, this research could contribute to the creation of an integrated model of early human development that takes biological, psychological, and social factors into account together. However, it remains crucial to continue interpreting the results with caution, as many of the observed correlations are complex and not clearly causal.