Stress Early in Pregnancy Linked to Fewer Baby Boys

Stress in early pregnancy can lead to shorter pregnancies, more pre-term births and fewer baby boys, a new study suggests.

Stress in the second and third months of pregnancy can shorten pregnancies, 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 that investigated the effect on pregnant women of the stress caused by the 2005 Tarapaca earthquake in Chile.

It has been known for a while that stress may affect the duration of pregnancy.

Now, Florencia Torche and Karine Kleinhaus of New York University, analysing birth of over 200,000 born between 2004-2006 in Chile found that the timing of the stress and the effect that stress might have an impact on the ratio of male-to-female births.

“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,” said Prof Torche who is Associate Professor of Sociology. “We were able to capture the developmental periods in which exposure to stress was most detrimental for either sex.”

The researchers found that women who experienced a severe quake 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 effect was most pronounced for female births; the probability of pre-term birth increased by 3.8% if exposure to the quake occurred in the third month, and 3.9% if it occurred in the second month. Comparable increases for males were insignificant.

Generally, there were more female than male live births. This finding may be related to previous research, which has found that male fetuses tend to grow larger than females and need more resources from the mother, and therefore are more likely to miscarry in times of stress. In addition, male fetuses may be less robust than females and may be less capable of adapting their development to a stressful environment in the womb.

The research 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.

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The above story is written by The Zestfulness Team from materials in the December 8, 2011 press release by the European Society of Human Reproduction and Embryology.

The article has been published in the December 7, 2011 online edition of Human Reproduction: Torche F, Kleinhaus K. Prenatal stress, gestational age and secondary sex ratio: the sex-specific effects of exposure to a natural disaster in early pregnancy. Hum Reprod (2011) doi:10.1093/humrep/der390.

More information

The March of Dimes has more about stress and pregnancy.