The relation between third-trimester coitus and adverse perinatal outcome was studied in 39 217 pregnancies from the Collaborative Perinatal Project. Coital frequencies reported at 28-29 weeks, 32-33 weeks, and 36-37 weeks were each positively related to subsequent length of gestation (p less than 0.001), both before and after adjusting for multiple covariates. Perinatal mortality was compared among women reporting different frequencies of coitus per month. The adjusted odds ratios (compared with women having no coitus) for women having coitus 0.1-2.0, 2.1-4.0, and greater than 4.0 times per month were 0.74, 0.89, and 0.87 for coitus at 28-29 weeks; 0.82, 0.82, and 0.99 for coitus at 32-33 weeks; and 1.15, 1.11, and 1.45 for coitus at 36-37 weeks. None of these odds ratios was statistically significant. Thus, in this data set there was no association between coitus in pregnancy and adverse outcome of pregnancy.