Umbilical cord length was measured in 9620 male and 9068 female infants and related to gestational age in order to construct a growth chart for umbilical cords. In addition to providing a standard for the US white population, these growth charts illustrate that umbilical cord length is widely divergent at the same gestational age, males have longer cords than females (P less than 0.0001), and umbilical cord growth continues throughout the third trimester.