- To determine maternal and neonatal complications among dichorionic and monochorionic twins with isolated midtrimester elevated maternal serum human chorionic gonadotropin (MShCG). MShCG was determined in 247 women with dichorionic twins and 32 women with monochorionic twins between 16-18 weeks gestation. Among the dichorionic twins 219 patients had MShCG < 2.5 MoM, 14 between 2.5-3.0 MoM and 14 above 3.0 MoM. Of the patients with monochorionic twins 15 had MShCG < 2.5 MoM, nine between 2.5-3.0 MoM and 8 above 3.0 MoM. All patients had maternal serum alpha fetoprotein < 2.5 MoM. Karyotype was normal among all neonates. Statistical analysis was performed with SPSS package. Patients with monochorionic twins had higher rates of cesarean section when MShCG was > 3.0 MoM (100% vs. 44%; p = 0.03) and of preterm delivery when MShCG was > 2.5 MoM (87.5% vs. 46.7%; p = 0.04). A non significant higher rate of small for gestational age (SGA) neonates was found when MShCG was > 2.5 MoM among first twin (37.5% vs. 13.3%; p = 0.08). In contrast, patients with dichorionic twins had higher rates of SGA neonates and low 1 minute Apgar scores in the second twin when MShCG was > 2.5 MoM (23.1% vs. 10%; p = 0.04, 15.4% vs. 11.9%; p < 0.01). A multivariate logistic regression model with forward stepwise selection was performed with SGA as outcome variable. The model included the following variables: MShCG, hypertensive disorders, gestational age at delivery, chorionicity, twin order, cesarean section (CS) and preterm delivery. MShCG levels were the only significant factor predicting SGA among bichorionic twins (OR 1.76, 95% CI 1.2-2.5). (1) Increased concentrations of MShCG are an independent risk factor for SGA among dichorionic twins. (2) MShCG > 2.5 MoM are associated with adverse maternal outcome among monochorionic twins.