OBJECTIVE: To measure the contribution of genetic factors to selected pregnancy complications, including miscarriage, twinning, hypertension-toxemia, and nausea-vomiting. METHODS: Information on 22,241 pregnancies of 8675 female twins or spouses of male twins was obtained by questionnaire from members of the population-based Norwegian Twin Panel. Comparisons of observed tetrachoric correlations were used to assess the importance of genetic influences on the variables examined. RESULTS: Pregnancy history information was provided by both members of 830 monozygotic and 902 dizygotic female twin pairs and by the spouses of both members of 459 monozygotic and 464 dizygotic male twin pairs. The incidence of twin pregnancy in general, and of opposite-sexed twins in particular, found among dizygotic twin women was nearly twice that observed for any other group. Monozygotic female twin pairs were more concordant than dizygotic female twin pairs for the occurrence of miscarriage, nausea or vomiting during pregnancy, and hypertension or overt toxemia. A similar pattern of twin similarity was observed for the use of certain medications during pregnancy including vitamins, aspirin, and nausea medication. CONCLUSIONS: Maternal genetic factors make an important contribution to a predisposition for dizygotic twinning, contribute to the risk of miscarriage, and appear to determine, in part, whether a woman experiences nausea-vomiting or hypertension-toxemia during pregnancy. In addition, health-seeking behaviors of women during pregnancy, as reflected by the use of several classes of medication, appear to be influenced somewhat by genetic factors.