Pregnant women are capable of influencing the timing of their babies' births, according to a study that shows fewer children are born on Halloween.

The results of an analysis of almost 2.5 million births in the US over 11 years contradicts the current medical orthodoxy that expectant mothers have no control over the timing of the delivery of their babies.

Dr Rebecca Levy of Yale School of Public Health, who led the study, said Halloween's associations with death, evil and skeletons might subconsciously put women off giving birth.

"The study raises the possibility that the assumption underlying the term 'spontaneous birth', namely, that births are outside the control of pregnant women, is erroneous," Dr Levy told New Scientist magazine.

She added that a connection between the state of mind of pregnant women and hormone levels could explain the link.

"We know that hormones control birth timing, and mothers do often express a desire to give birth on a certain day," she says. "But the process that allows those thoughts to potentially impact the timing, we don't know."

Dr Levy and colleagues analysed data from birth certificates for all births in the US that took place within one week on either side of Valentine's Day and Halloween between 1996 and 2006. They found the likelihood of women giving birth on Valentine's Day was on average 5% higher than on other days during the week before or the week after.

It was 3.6% higher for natural, non-induced births and 12.1% higher for Caesarean section births.

The chance of deliveries occurring on Halloween was on average 11.3% lower than during the days in the week before and after. This broke down to 5.3% lower for natural, non-induced births, and 16.9% lower for Caesareans. The results are published in the journal Social Science & Medicine.

Dr Levy said more research was needed to characterise precisely how particular states of mind affected physiology to speed up or delay birth.

There has been anecdotal evidence from partners of members of the military suggesting that when fathers are due to return from postings away from home close to the date of birth, their babies sometimes "wait" until their return before being born.

A 2003 study carried out in Taiwan showed increases in Caesarean births on auspicious days and decreases on inauspicious days of the Chinese lunar calendar.