Percentile curves representing intrauterine growth of monochorionic and dichorionic twins were constructed from the birth weights of liveborn infants at gestational ages ranging from 24 to 42 weeks. The median weights of twins were progressively lower than median weights for single-born infants during late gestation. Monochorionic twins were both smaller and had a greater intrapair variation in birth weight than dichorionic pairs. Intrauterine growth charts prove useful in studying factors influencing twin growth during both the prenatal and postnatal periods.