Stress in the second and third months of pregnancy can shorten the process, increase the risk of pre-term births, and may affect the ratio of boys to girls being born, leading to a decline in male babies. These are the conclusions of a study by NYU researchers that investigated the effect on pregnant women of the stress caused by the 2005 Tarapaca earthquake in Chile.
Although it has been known for a while that stress may affect the duration of pregnancy, until now, no study has looked 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. The research, published online in Europe’s leading reproductive medicine journal, Human Reproduction, provides answers to these questions and also suggests that it is exposure to stress itself rather than other factors that can often accompany or cause stress, such as poverty, that appears to affect pregnancy.
The study was conducted by Florencia Torche, an associate professor in NYU’s Department of Sociology, and Karine Kleinhau, an assistant professor in the Departments of Psychiatry, Environmental Medicine, and Obstetrics and Gynecology at the NYU Langone Medical Center.
They analyzed birth certificates of all babies born between 2004 and 2006 in Chile, where there were more than 200,000 births a year. The birth records provided information on gestational age at delivery, sex, weight, and height of the baby and whether any medical attention was required. They 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.
“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 epicentre,” says Torche.
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. The researchers found that 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.
“In terms of implications, it is clearly unrealistic to recommend avoiding natural disasters,” Torche concludes. “However, this research suggests the need to improve access to health care 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.”