Stress in Early Pregnancy Linked to Prematurity, Fewer Boys

Ricki Lewis, PhD

December 09, 2011

December 9, 2011 — A comparison of pregnancies a year before and during a major earthquake associates the stress of the experience to an increase in female preterm births and an overall decrease in male births. The report was published online December 8 in Human Reproduction.

Past studies have shown that stress during pregnancy can shorten gestation and possibly alter the secondary ratio of male to female births. However, past studies were unable to assess the effect of the timing of stress relative to gestation. The month-by-month comparison in the new study pinpoints the second and third months as a vulnerable time in gestation.

Florencia Torche, PhD, and Karine Kleinhaus, MD, MPH, from New York University, New York City, used regression models to compare pregnancy outcomes among women who lived through the 2005 Tarapaca earthquake in Chile with those of women who had been at the same stage of pregnancy a year earlier, as well as with those of women pregnant at the same time as the earthquake, but living in unaffected areas. The researchers analyzed birth certificates from 2004 to 2006 from the Chilean Ministry of Health.

The investigation was unique in considering both duration of pregnancy and secondary sex ratio. In addition, the use of a natural disaster enabled the researchers to separate the physical stress from other sources of stress during pregnancy, such as poverty.

Exposure to the earthquake during the second and third months led to a significant decrease in gestational age at birth and an increase in preterm births, particularly among female offspring. The probability of preterm births of girls increased by 0.038 (95% confidence interval, 0.005 - 0.072) with stress exposure in the second month and by 0.039 (95% confidence interval, 0.002 - 0.075) with stress exposure in the third month. The increase in preterm births among boys was not significant. "These findings are striking because in our unexposed Chilean population, as in most populations, males are more likely to be born preterm," the researchers write.

Pregnancies exposed to the earthquake during the second month were on average 1.3 days shorter than those in unaffected counties, and those exposed in the third month were 1.9 days shorter. Nine exposed women in 100 gave birth early compared with the usual figure of 6 women in 100.

After accounting for the sex-specific effect on gestational age, the researchers detected a significant decline in the male-to-female ratio for third-month exposure of 5.8% (−0.058; 95% CI, −11.3% to −0.3%). They hypothesize that this apparent "male culling" during times of stress is consistent with the concept in evolutionary biology that selective abortion of males enables pregnant women to conserve resources.

"Our findings...suggest that stress may affect the viability of male births. In contrast, among female conceptions, stress exposure appears not to affect the viability of the conception but rather, the length of gestation," concludes Dr. Torche in a news release.

The investigators hypothesize that an impaired placenta may underlie the findings, via actions of the stress hormone cortisol. They call for further studies to see whether the placentas of female fetuses are more responsive to maternal stress hormones than are the placentas of male fetuses.

The authors have disclosed no relevant financial relationships.

Hum Reprod. Published online December 8, 2011. Abstract

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