The purpose of this prospective cohort study of twins and triplets was to evaluate perinatal and early childhood outcomes through 18 months of age. The study population included 141 twin pregnancies (282 twin children) and 8 triplet pregnancies (24 triplet children) recruited between May, 1996 and June, 2001. Mothers of triplets versus twins were significantly more likely to have infertility treatments, to be overweight or obese before conception, to be admitted antenatally, and to deliver by cesarean section. Length of gestation for triplets was significantly shorter (–2.31 weeks, p < .0001), and more likely to be less than 35 weeks (Adjusted Odds Ratio [AOR] 9.38, 95% confidence interval [CI] 3.22–27.29). Average birthweight for triplets was significantly lighter (–495 grams, p < .0001), and more likely to be low birthweight (AOR 11.38, 95% CI 3.11–41.61). Triplets were also more likely to be admitted to neonatal intensive care (AOR 7.97, 95% CI 2.13–29.77), to require mechanical ventilation (AOR 5.67, 95% CI 2.05–15.65), to develop respiratory distress syndrome (AOR 12.50, 95% CI 3.89–40.20), or a major morbidity (retinopathy of prematurity, necrotizing enterocolitis, ventilator support, or grade III or IV intraventricular hemorrhage, AOR 5.67, 95% CI 2.05–15.65). Weight, length, and head circumference was significantly smaller at birth for triplets compared to twins, and these differences remained through 18 months of age, along with lower mental developmental scores at the oldest age. Compared to twins, triplets have greater neonatal morbidity, and through 18 months of age lower mental and motor scores, slower postnatal growth and more residual stunting, particularly of length and head circumference.