Undernutrition during pregnancy is associated with detrimental pregnancy and neonatal outcomes, which can have long-term implications for the infant. Hyperemesis gravidarum may severely limit nutritional intake. The aim of this study was to investigate the effect of hyperemesis on pregnancy and neonatal outcome, particularly gestation length and infant size at birth. Seventy-five prospectively recruited women admitted to a tertiary level hospital in Auckland, with hyperemesis gravidarum between March 2003 and October 2005, were compared to 142 controls matched for age, parity, ethnicity and expected date of delivery. Data were obtained from electronic records and analysed by Student's t-test, χ2, Wilcoxon, Fisher's exact tests and linear regression. Length of gestation, birth weight and crown-heel length were not different between participants and controls. Infants born to women with hyperemesis gravidarum had smaller head circumferences (Z-score mean (s.d.) 0.02 (0.16) v. 0.43 (0.11), P = 0.04 in all infants and -0.02 (1.24) v. 0.48 (1.29), P = 0.01 in-term infants). This study found hyperemesis gravidarum to be associated with smaller head circumferences in offspring. Given the reported associations between smaller head circumference at birth and lower cognitive ability and higher risk of cardiovascular disease in later life, further study is necessary to confirm these results and to determine whether there are any long-term implications for the offspring.