Stress during pregnancy 'raises risk of premature birth'

Researchers at New York University have found that exposure to stress can shorten the length of pregnancy, making it more likely that babies will be born early and for boys to be miscarried.

It is the first time stress has been shown to affect the balance between the sexes, known as the sex ratio, which normally favours an excess in the number of boys being born, the 'Daily Mail' reported.

The latest findings suggest the extra risk to boys starts even earlier, in the womb. In fact, the study involved investigating the effect on pregnant women of stress caused by the 2005 Tarapaca earthquake in Chile.

For their study, the researchers analysed birth certificates of all babies born between 2004 and 2006 in Chile, where there are 200,000 births a year. The magnitude of the quake was measured at 7.9, classified as "disastrous".

The researchers found that women who lived closest to the 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 pregnancies of women exposed to the earthquake in the second month of pregnancy were on average 1.3 days shorter than those in unaffected areas of Chile. The pregnancies of those exposed in the third month were almost two 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 per cent, meaning more than nine women in 100 delivered their babies early.

The researchers found a decline in the sex ratio among those exposed to the earthquake in the third month of 5.8 per cent, meaning fewer boy babies survived to delivery.

Prof Karine Kleinhaus, who led the study, 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 per cent 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."