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 stated.
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.
