NEW YORK (Reuters Health) - Women who conceive in the springtime may be more likely to deliver before 37 weeks gestation than women who conceive at other times of the year, two researchers from the University of Pittsburgh Medical Center have found.

Drs. Lisa M. Bodnar and Hyagriv N. Simhan analyzed the records of 75,399 women who delivered at Magee-Women's Hospital over the 10-year period 1995 to 2005. They grouped the women by season of last menstrual period, used to estimate time of conception.

"Preterm birth was most common among women who conceived in the springtime and was least common in women who conceived in the summer," Bodnar noted in a telephone interview with Reuters Health.

Specifically, the rate of preterm birth was 8.4 percent for summer conception, 8.8 percent for fall conception, 9.1 percent for winter conception, and 9.2 percent for spring conception.

The rate of very preterm birth, or birth before 32 weeks gestation, was also lowest with summer conception (2.0 percent), with steadily increasing rates for fall conception (2.3 percent), winter conception (2.5 percent) and spring conception (2.7 percent).

"This is pretty intriguing data given that it's such a large sample size and over such a large time span," Bodnar said. "We were interested in looking at conception because the roots of preterm birth probably occur long before a woman ever develops symptoms of preterm birth where she goes into labor or her water breaks."

Bodnar and Simhan point out that while the current study does not address reasons for seasonal variation in preterm birth rates, it's possible seasonal allergens and viral infections are involved as well as perhaps seasonal dietary changes, sunlight exposure, and exercise habits.

"Everyone has heard that we 'put on an extra layer' in winter, and micronutrient intake shifts with the seasons," Simhan elaborated. "We also know that inflammation plays a role. It could be that becoming pregnant when the immune system is primed by viral and bacterial exposures may be a factor weeks down the road."

Simhan presented the study findings this week in San Francisco at the meeting of the Society of Maternal-Fetal Medicine.