Stress in the second and third months of pregnancy may affect the ratio of boys to girls being born, leading to a decline in the number of baby boys. The findings from a new study of pregnant women following the 2005 Tarapaca earthquake in Chile also confirms previous findings that stress can shorten pregnancies and increase the risk of pre-term births. The research, published online in the journal Human Reproduction, is the first to look at the impact of both the timing of the stress and the effect that stress might have on the ratio of male-to-female births. For the study, scientists analyzed birth certificates of all babies born between 2004 and 2006 in Chile, where there were more than 200,000 births a year. The records included information on gestational age at delivery, sex, weight, and height of the baby and whether any medical attention was required. The records also included information on the mother’s age, marital status, whether or not she had been pregnant before, and in which of the 350 counties in Chile she lived—information that provided very specific information about how exposed the mothers were to the effects of the earthquake, based on how close they lived to the epicenter. “Looking at information on gestational age at the time of the earthquake in a large, unselected group of women, enabled us to determine the risk for specific birth outcomes by gestational age of exposure to a stressor, which, because it was a natural disaster, was experienced by all at the same time, although in varying degrees of severity, depending on how close they lived to the epicenter,” says Florencia Torche, an associate professor of sociology at New York University. ““We were able to capture the developmental periods in which exposure to stress was most detrimental for either sex.” The earthquake measured 7.9 on the moment-magnitude scale (the successor to the Richter scale), which is classified as “disastrous.” The areas most affected were the cities of Iquique and Alto Hospicio, and the surrounding towns. Women who experienced a severe quake (because they lived closest) during their second and third months of pregnancy had shorter pregnancies and were at higher risk of delivering pre-term (before 37 weeks gestation). The pregnancies of women exposed to the earthquake in the second month of pregnancy were on average 0.17 weeks (1.3 days) shorter than those in the unaffected areas of Chile. The pregnancies of those exposed in the third month were 0.27 weeks (1.9 days) shorter. Normally, about six in 100 women had a pre-term birth, but among women exposed to the earthquake in the third month of pregnancy, this rose by 3.4 percent, meaning more than nine women in 100 delivered their babies early. The effect was most pronounced for female births; the probability of pre-term birth increased by 3.8 percent if exposure to the quake occurred in the third month, and 3.9 percent if it occurred in the second month. In contrast, there was no statistically significant effect seen in male births. Because the stress of the earthquake had greater effects on pre-term births in girls rather than boys, the researchers had to make adjustments for this when calculating the effect of stress on the sex ratio: the ratio of male to female live births. They found that there was a decline in the sex ratio among those exposed to the earthquake in the third month of gestation of 5.8 percent. “Generally, there are more male than female live births. The ratio of male to female births is approximately 51:49 – in other words, out of every 100 births, 51 will be boys. Our findings indicate a 5.8 percent decline in this proportion, which would translate into a ratio of 45 male births per 100 births, so that there are now more female than male births. This is a significant change for this type of measure,” says Karine Kleinhau, an assistant professor of psychiatry, environmental medicine, and obstetrics and gynecology at NYU. Previous research has suggested that in times of stress women are more likely to miscarry male fetuses because they grow larger than females and therefore require greater investment of resources by the mother; they may also be less robust than females and may not adapt their development to a stressful environment in the womb. “Our findings on a decreased sex ratio support this hypothesis and suggest that stress may affect the viability of male births,” Torche says. “In contrast, among female conceptions, stress exposure appears not to affect the viability of the conception but rather, the length of gestation.” The findings could be explained by what happens to the placenta—which sets the duration of the pregnancy—and the effect of the stress hormone cortisol on the placenta’s function. “In terms of implications, it is clearly unrealistic to recommend avoiding natural disasters,” Torche says. “However, this research suggests the need to improve access to healthcare for women from the onset of pregnancy and even before conception. Obviously this will not reduce the exposure to stress, but it may provide care, advice, and tools that would allow women to cope with stressful circumstances. “A separate implication has to do with our ability to use a natural experiment (the earthquake) to isolate the effect of stress from factors that commonly go with it. In particular, researchers have long suggested that poverty is bad for health outcomes because of the stress it elicits. “This is very plausible, but it is difficult to disentangle the effect of stress alone from the effect of other factors associated with poverty, such as nutritional deprivation and poor housing, which could also have an independent impact on women’s health and the outcome of their pregnancies. This makes it difficult to ascertain whether stress itself does, indeed, matter. Our research provides strong evidence that it does.”