Intrauterine growth in multiple pregnancies in relation to fetal number, chorionicity and gestational age.

OBJECTIVES To examine birth weight in dichorionic and monochorionic twins and trichorionic triplets, and to estimate the relative independent importance on growth of fetal number, chorionicity and gestational age. METHODS Four groups of pregnancies were examined: 12,816 singleton pregnancies, 302 monochorionic twin pregnancies, 569 dichorionic twin pregnancies, and 97 trichorionic triplet pregnancies. Chorionicity was determined at 10-14 weeks on the basis of the presence or absence of the lambda sign in twins, and by examining the ipsilon zone in triplets. The relationship between birth weight and gestational age in singletons was established, and using this equation the expected mean birth weights for all gestational ages were calculated. For each case in all groups, the difference between the observed birth weight for each fetus and the appropriate normal mean for gestation in singletons was calculated (Z-score). Multiple regression analysis was used to examine the independent contribution of gestational age, number of fetuses and chorionicity in the prediction of actual birth weight and birth weight Z-score. RESULTS Birth weight Z-score was significantly lower than the expected mean in singletons for dichorionic twins, monochorionic twins and trichorionic triplets (t = 15.4, P < 0.0001, t = 21.7, P < 0.0001 and t = 19.9, P < 0.0001, respectively). Furthermore, the reduction in expected birth weight was significantly greater for monochorionic twins and trichorionic triplets compared with dichorionic twins (t = 6.3, P < 0.0001 and t = 7.8, P < 0.0001, respectively). Multiple regression analysis demonstrated that number of fetuses, presence of a monochorionic placenta and gestational age were independently associated with birth weight Z-score, the strongest effect being fetal number, followed by monochorionicity (t = - 23.4, P < 0.0001, t = - 8.3, P < 0.0001 and t = - 4.9, P < 0.0001, respectively). CONCLUSIONS The finding that monochorionic twins were of lower adjusted birth weight than dichorionic twins, and the significant independent effect of chorionicity on birth weight suggest that monochorionic placentation in itself has an effect on intrauterine growth. The effect of fetal number independent of chorionicity is demonstrated by the lower birth weight of trichorionic triplets compared with dichorionic twins.