Summary. The outcome of pregnancy was compared in 1065 booked and 1216 unbooked (emergency admissions) Hausa–Fulani primigravidae, half of them aged 10–16 years. The height of every woman was measured only once; this was either in early pregnancy in the booked primigravidae (range 122–176 cm) or at delivery in the unbooked primigravidae (range 130–186 cm). Mean fetal birthweights differed very significantly, being 2.91 kg and 2.76 kg for the booked and unbooked primigravidae respectively. The prevalence of cephalopelvic disproportion and of delivery by caesarean section or embryotomy were 9% and 11% respectively for the booked and 30% and 30% for the unbooked. There were two reasons why the booked primigravidae were less likely to develop cephalopelvic disproportion despite producing bigger babies; one was selection bias in the unbooked and the other (and more important) was growth of a teenage primigrivadae during pregnancy.