Intrauterine growth restriction in twin pregnancies: incidence and associated risk factors.

We sought to estimate the association of several maternal risk factors with intrauterine growth restriction (IUGR) in twin pregnancies. This is a case-control study of 313 patients with twin pregnancies delivered greater than 24 weeks between June 2005 and January 2010. We used three definitions of IUGR: (1) either twin with a birth weight < 10th percentile for gestational age; (2) either twin with a birth weight < 5th percentile for gestational age; and (3) birth-weight discordance ≥ 20%. Using each definition of IUGR, we estimated the association between IUGR and maternal age, weight, monochorionicity, in vitro fertilization, pregnancy reduction, thrombophilia, hypertension, and diabetes. Overall, 47% of patients delivered at least one twin with a birth weight <10th percentile, 27% of patients delivered at least one twin with a birth weight <5th percentile, and 16% of patients had birth-weight discordance of ≥20%. Using any of these three definitions for IUGR in twin pregnancies, there was no significant association between IUGR and any of the risk factors examined. This remained true when we excluded all patients who delivered <34 weeks. IUGR is very common in twin pregnancies. However, in twin pregnancies, IUGR cannot be predicted by maternal risk factors.