Rebecca Levy at Yale School of Public Health and her colleagues, analyzing 1.8 million U.S. birth records from 1996 to 2006, found birth rates dropped by 11.3 per cent on Oct. 31 when compared with the weeks before and after the holiday, NewsScientist.com reported Monday.
"We know that hormones control birth timing, and mothers do often express a desire to give birth on a certain day," Levy said. "But the process that allows those thoughts to potentially impact the timing, we don't know."
A psychological influence over hormonal activity may be at work, she said.
"Halloween can have pretty scary imagery of skeletons, death, devils, monsters," she said. "It's possible that death imagery is particularly salient as people are thinking about birth. [Perhaps] it evokes fear on some level."
The researchers also analyzed birth rates around Valentine's day, associated with feelings of love, and found a 5 percent spike in births on Feb. 14 compared with the two-week window surrounding the date.
"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," Levy said.