Prenatal care and pregnancy outcome in an HMO and general population: a multivariate cohort analysis.

We studied the use of prenatal care and pregnancy outcome in 4,148 deliveries among members of a well-established health maintenance organization (HMO) and 19,116 births among the 1973-1974 White birth cohort in the Portland, Oregon area. Mothers in the HMO were almost one year older on the average, slightly better educated, and less frequently unmarried, but had virtually identical past pregnancy histories when compared with the general population cohort. HMO members began prenatal care one month later and had three fewer visits than the general population (p less than .01); 78 per cent of the general population and only 64 per cent of HMO members began prenatal care in the first trimester (p less than .01). With maternal risk held constant, low birthweight, neonatal mortality, and infant mortality were 1.5 to 5 times greater with late, less frequent prenatal care than with early, frequent care. Multivariate analysis demonstrated a positive relationship between prenatal care and birthweight. Although this relationship was independent of risk factors recorded on birth certificates, it is not necessarily a causal relationship. Unadjusted prematurity, neonatal and infant mortality rates did not differ between the HMO and general populations. Multivariate analyses indicated that, independent of all maternal risk factors, HMO membership was associated with an increase of 30 grams in the predicted birthweight (P less than .01), but had no effect on mortality. The data suggest that, in Portland, Oregon, pregnancy outcome for HMO members is comparable to that of the general population.