Renewed interest in less costly, less technologically oriented obstetrical care requires the accurate selection of women who may safely benefit from such alternatives. The distribution of complications of labor and delivery, among healthy women who had had uncomplicated pregnancies, was investigated by studying data from more than 240,000 birth certificates filed with the New York City Department of Health during the period 1972-1974. Complications of labor or delivery were reported for 21.0 per cent of births following apparently uncomplicated antepartum courses. Significantly higherrates of recorded complications were found for women who had initiated prenatal care earlier in pregnancy, for private patients as opposed to general service patients, for white patients compared with nonwhite patients, for married women than for unmarried women, and for better educated patients as opposed to those with less schooling. Labor and delivery complication rates were also noted to rise with maternal age. The positive association between earlier prenatal care and higher complication rates was found within all service, racial, marital, educational and age categories, and appeared to be independent of these variables. Factors which may contribute to these unexpected patterns, and further research to clarify them, are discussed.