Spontaneous reduction of multiple pregnancy: incidence and effect on outcome.

Objective: Our objective was to determine the incidence of spontaneous reduction in multiple pregnancies during the first 12 gestational weeks and determine the outcome of the surviving fetuses.
Study Design: Analysis of prospectively collected ultrasound and birth information on 709 multiple and 5962 singleton pregnancies conceived at a private infertility clinic.
Results: Spontaneous reduction of one or more gestational sacs and or embryos occurred before the 12th week of gestation in 36% of twin (95% CI, 32%-40%), 53% of triplet (95% CI, 44%-61%), and 65% of quadruplet (95% CI, 46%-85%) pregnancies. Reduction was less frequent after ovulation induction than after spontaneous ovulation. In general, pregnancy duration and birth weight were inversely related to the initial gestational sac number irrespective of the final birth number.
Conclusions: More than 50% of patients with 3 or more gestational sacs had spontaneous reduction before 12 weeks. The surviving fetuses weighed less and were born earlier than unreduced pregnancies with the same initial number of fetuses.