The umbilical cord is vulnerable to a number of insults that may alter cord morphology, diminish cord flow, and ultimately compromise fetal nutrition. Thus, an investigation of the underlying mechanisms of the development of cord morphology and possible pathologies associated with it may provide insight regarding fetal growth in the intrauterine environment and have an impact on later development of the child. To our knowledge, this study, which included 11 980 twins, is the first to report the relative contribution of genes and environment in the development of the cord. Umbilical cord length, insertion, knots, twisting, and number of vessels were examined by trained midwives at birth. Means and percentages of cord characteristics by twin zygosity/chorionicity and gender were calculated. ANOVA and chi-square tests were performed to calculate discordance in cord morphology between dizygotic (DZ), monozygotic monochorionic (MZMC), and monozygotic dichorionic (MZDC) twins. Univariate genetic models were fit to the umbilical cord characteristics to investigate the genetic and environmental influences on umbilical cord morphology. Mainly nonshared environmental but also genetic factors influence umbilical cord morphology. In MZMC male and female twins, a peripheral/marginal cord insertion was significantly (P < 0.01) more prevalent compared to MZDC and DZ male and female twins, respectively. In MZMC male twins, clockwise twisting was significantly (P = 0.02) less frequent compared to DZ twins. Environmental and genetic factors influence cord morphology and pathology. Twin members can experience environmental influences that are not shared between them even in that very early stage of in utero life.