Increased incidence of high-order and singleton conceptions after ovulation induction in winter

The possible relationship between the season of conception after ovulation induction and high-order multifetal pregnancies, was investigated. From 1975 to 1989, 111 high-order multiple births after ovulation induction were recorded at the Chaim Sheba Medical Center. This group was compared with 142 singleton pregnancies that resulted from induction of ovulation during 1989. Composite monthly cohorts of high-order multifetal pregnancies were constructed for each month of the year, and the probability of such pregnancies was estimated. The period of ovulation induction and the day of presumed conception were noted. A statistically-significant increase in the probability of high-order and singleton conceptions occurred during the winter. Our observation indicates a seasonal pattern in high order and singleton conceptions after ovulation induction.