According to a study published online in Europe’s leading reproductive medicine journal Human Reproduction on December 8th, mothers who are stressed during the second and third trimester of pregnancy can reduce the length of their pregnancy and increase the risk of their unborn child being born prematurely. In addition, stress may also affect the ratio of boys to girls born, leading to a decline in male babies. The study examined the effect stress caused by the 2005 Tarapaca earthquake in Chile had on pregnant women.

Even though researchers know that stress may shorten pregnancy, until now, no investigation has examined the impact of the timing of the stress and the effect stress may have on the ratio of boys to girls being born.

These questions are covered in the new study which also indicates that its exposure to stress itself instead of other factors that can often contribute or cause stress, such as poverty, that seems to affect pregnancy.

In Chile between 2004-2006, there were more than 200,000 births per year. Birth certificates of all babies born during this duration were examined by Professors Florencia Torche (PhD) and Karine Kleinhaus (MD, MPH), of New York University (New York, USA).

Each birth record included data on gestational age at delivery, weight, height, and sex of the baby as well as if any medical attention was needed. In addition, the records provided data on the mother’s age at delivery, previous pregnancies, if any, marital status, and in which of the 350 counties in Chile she resides. This data provided the team with extremely specific data on how much the mothers were exposed to the effects of the earthquake, based on how close to the epicenter they lived.

Prof Torche who is Associate Professor of Sociology, explained:

“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. We were able to capture the developmental periods in which exposure to stress was most detrimental for either sex.”

Iquique and Alto Hospicio, and the surrounding towns were the areas most effective by the “disastrous” earthquake which measured 7.9 on the moment-magnitude scale (the successor of the Richter scale). The team discovered that women who lived closest to the epicenter of the earthquake during their second and third trimesters of pregnancy had shorter pregnancies and were at an increased risk of delivering prematurely (before 37 weeks gestation).

On average, women exposed to the earthquake in their second trimester delivered their babies 0.17 weeks (1.3 days) earlier than women in unaffected areas of the country and those in their third trimester delivered their babies 0.27 weeks (1.9 days) earlier. Usually, approximately 6 in 100 women had a pre-term birth, however this increased by 3.4% (9 in 100 women) in women exposed to the earthquake in their third trimester.

The effect was most notable for female births; the probability of pre-term birth rose by 3.8% if the mother was exposed to the earthquake during her third trimester, and 3.9% if it occurred in the second trimester. In male births there was no statistically considerably effect observed.

When calculating the effect of stress on the sex ratio: the ratio of male to female live births, the team had to make adjustments as the stress of the quake had a more significant effect on pre-term births in girls than boys. They discovered there was a decline of 5.8% in the sex ratio among those exposed to the quake in the third trimester.

Prof Kleinhaus, who is Assistant Professor of Psychiatry, Obstetrics & Gynecology, and Environmental Medicine, said:

“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% 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.”

Previous studies had indicated that women who are stressed are more likely to miscarry male fetuses as they grow larger than females, thus requiring more resources by the mother. In addition, they may be not as strong as females and might not adapt their development to a stressful environment in the womb.

Prof. Torche, explained:

“Our findings on a decreased sex ratio support this hypothesis and suggest that stress may affect the viability of male births. In contrast, among female conceptions, stress exposure appears not to affect the viability of their conception but rather, the length of gestation.”

The investigators state that the placenta, which controls the duration of the pregnancy, and the effect of the stress hormone cortisol on the placenta’s function, may be possible mechanisms to explain their discoveries.

Prof Torche concluded:

“In terms of implications, it is clearly unrealistic to recommend avoiding natural disasters. 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 the 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.”

Written by: Grace Rattue