Babies born before arrival at hospital. OBJECTIVE--To establish the prevalence of babies born before arrival at two local hospitals. To identify women at risk of giving birth before arrival, and the morbidity and mortality associated with such births. DESIGN--A case control study. Each baby born before arrival and its mother were compared with the next born in the hospital (random control), and one matched for gestation and birthweight, together with their mothers. SETTING--Two maternity units serving East Birmingham and Solihull. SUBJECTS--All babies (and their mothers) born before arrival at these hospitals from January 1983 to December 1987. MAIN OUTCOME MEASURES--Perinatal mortality rates, patterns of perinatal morbidity, demographic, social and obstetric features of the mothers. RESULTS--137 (0.44%) of 31,140 consecutive births were before arrival at hospital (BBA group). The perinatal mortality rate in the BBA group was 58.4/1000 (8 deaths) compared with 10.1/1000 for all inborn babies (relative risk 5.8, 95% confidence interval 2.9-11.4). In the BBA group the mean birthweight of 3008 g was 212 g (95% CI 50-374 g) less than that in the random control group; the mean gestation of 266 days was 10 days less (95% CI 5.9-14.1 days) than in the random control group. Hypothermia was the commonest morbidity. Women delivered before arrival tended to be either multigravid inner city Asians living a long way from the hospital or unmarried unbooked younger white Europeans. CONCLUSIONS--The high perinatal mortality was related to immaturity and low birthweight, rather than to birth before arrival itself. Although groups of mothers at risk of delivery before arrival can be identified more information is needed to establish whether additional antenatal care would be beneficial for these women and their babies. 