SMFM: Timing of Conception Linked to Risk of Preterm Birth

February 7, 2007

SAN FRANCISCO -- The season of conception may be a factor in the risk for preterm delivery, researchers will report here this weekend.

SAN FRANCISCO, Feb. 7 -- The season of conception may be a factor in the risk for preterm delivery, researchers will report here this weekend.

In more than 75,000 births in a single-Pittsburgh hospital over a 10-year period, 9.2% of the preterm deliveries occurred among women who conceived in the spring compared with a rate of 8.4% among women who conceived during summer months (P

Data from 75,399 deliveries from 1995 though 2005 were included. All deliveries occurred at Magee-Women's Hospital of the University of Pittsburgh Medical Center.

The season of conception was calculated using the date of last menstrual period. a date historically used to estimate conception.

Among the findings:

  • Preterm birth (at less than 37 weeks gestation) at was least common among women with last menstrual period in summer a summer.
  • The preterm delivery rate was 8.8% for babies conceived in the fall and 9.1% for babies conceived during the winter months.
  • After adjusting for race and age, prevalence odds ratios (95% confidence interval) were 0.91 (0.84, 0.97) for summer, 0.94 (0.87, 1.0) for fall, and 0.99 (0.0.92, 1.06) for winter compared with spring.
  • Preterm birth at less than 32 weeks also occurred less often among women with who had their last menstrual period in the summer and fall than in than winter or spring (P