First trimester interfetal size variation in well-dated multifetal pregnancies.

Sonographic measurements of multifetal pregnancy for dating and growth are considered identical to singleton pregnancies until the second trimester. Observations in 57 patients having triplets or more, who were referred for first trimester multifetal pregnancy reduction, suggest considerable size variability both within an individual pregnancy and among pregnancies of the same gestational ages (GAs). These data are unique because the GAs are precisely known, because the pregnancies were established by assisted reproductive technology. We observed a greater standard deviation at 12 and 13 weeks of GA. We also observed significant interfetal variation within an individual pregnancy. Because multifetal pregnancy reduction was performed, follow-up of any given fetus to term was not possible. We conclude that for high-order multiple gestations: (1) interfetal size variability begins much earlier than often stated; (2) differences in the degree of variability can be observed among different pregnancies of the same gestational age and within the same pregnancy, and (3) there is a trend for increasing variability with increasing gestational age.