Stress, that feeling of unrelenting life pressure, gets under the skin and affects the body. There is ample evidence for this process, particularly with respect to chronic conditions such as obesity and cardiovascular disease. Meanwhile, other research has identified associations between women’s level of stress during pregnancy and risk for giving birth too early — that is, having a baby born preterm, equal to or fewer than 37 weeks gestation, which is linked to higher rates of infant mortality and to physical and mental disorders, such as attention-deficit hyperactivity disorder and anxiety. In this work, what kind of stress matters is unclear. Is it psychological stress, the sense of being overwhelmed and unable to keep up with life’s demands, that is related to poor birth outcomes? Or, is it stress that has gotten into the body and is identifiable on the physical level, say evidenced in higher blood pressure?

Remarkably, maternal prenatal stress also relates to which sex is born. In epidemiological studies of population-level data, traumatic events such as President Kennedy’s assassination and the 9/11 terrorist attacks were associated with a shift in what is called the Secondary Sex Ratio such that fewer males were born compared to females, a reverse from the species-typical, population norm of 105 males to 100 females. Here too, what kind of stress matters is unclear. What is known is that the male babies are more vulnerable to an adverse, stressed, prenatal environment, and that evolutionary science might help us understand this phenomenon: for a species such as ours to survive, relatively few males are needed for multiple conceptions whereas many females are needed as most commonly it is just one pregnancy per baby. When environmental conditions are challenging, vulnerable males are less likely to be born (women may experience pregnancy losses that are disproportionately male) culling out the weak so that the males who make it are the strong ones (all the better for species long-term survival).

The womb is an influential first home.

As we are keenly interested in women’s emotional state during pregnancy, how it affects them, and potentially their future child, we wanted to try to answer several questions: what kind of stress matters in relation to child outcomes? And if we characterized well what kind of stress matters, could we detect differences in sex at birth in a small, non-population-based study?

We examined 27 indicators of psychosocial, physical, and lifestyle stress collected from questionnaires, diaries, and daily physical assessments of 187 otherwise healthy pregnant women, ages 18 to 45. Using a data-driven approach to our analyses, we found that about 17% of the women were psychologically stressed, with clinically meaningful high levels of depression, anxiety, and perceived stress. Another 16% were physically stressed, with relatively higher daily blood pressure and greater caloric intake compared with other healthy pregnant women. The majority (nearly 67%) were healthy, truly healthy. Surprisingly, what most differentiated the groups was endorsement of social support in one’s life, a sense of belong to others and having others to carry out tasks when that is needed; compared to the healthy group, the two stressed groups reported low levels. Social support may be a target for effecting intervention. Helping pregnant women find support, and identify what is impeding the development of it, may go a long way to reducing stress effects in women and their future children. 

In our results, preterm birth rates varied dramatically according to maternal stress group. Physically stressed versus healthy group infants were born 1.5 weeks earlier with 22% compared to 5% born preterm. For the psychologically stressed, 9% were preterm. In the U.S., the average preterm birth rate is between 8-10%. Our results suggest that addressing women’s physical markers of stress, and emotional stress, could have a significant impact on lowering the nation’s preterm birth rate.

Our study also found that pregnant women experiencing physical and psychological stress were less likely to have a male baby. Compared to the typical 105:100 male-to-female ratio, in this study, the sex ratios in the physically and psychologically stressed groups favored girls, with male-to-female ratios of 4:9 and 2:3 for physically and psychologically stressed women, respectively, and higher in the healthy group 23:18. Since sex of a baby is typically like a flip of a coin (50/50), women in the stress groups likely are not having male babies while enrolled in our study due to the loss of males in prior pregnancies, often without even knowing they were pregnant. Of course, there is nothing preferable to a male or female baby in our lifetimes (versus from an evolutionary perspective). These data demonstrate how malleable birth outcomes are, what a woman experiences affects her child, even before birth, even his/her sex. As the father of prenatal programing studies David Barker once said, the womb is an influential first home.