AIMS To review the accuracy with which obstetric information on gestation is recorded in the neonatal records; and the reliability of the methods used for assessing gestational age after birth. METHODS Service information on all babies born in 1989, and research information on all babies of <32 weeks gestation born in the Northern Region in 1990–91, were reviewed to determine the accuracy with which antenatally collected information had been recorded in the neonatal records after birth. A prospective study was also mounted to assess how reliably paediatric staff could assess the gestational age of babies born to mothers with certain obstetric dates under service conditions. Paediatric residents looked at 347 babies of > 32 weeks gestation, and senior staff looked at 105 babies of < 30 weeks gestation. RESULTS The best techniques for estimating gestation immediately after birth were only half as accurate (95% CI ± 17 days) as estimates based on antenatal ultrasound at 15–19 weeks gestation. Assessments that relied on the tone, posture, and appearance of the baby at birth in those of < 32 weeks gestation were less reliable than assessments based on a retrospective review of when various reflex responses first appeared. They also tended to overestimate true gestation. Antenatal information of high quality was ignored, and arithmetic and transcription errors were introduced during the transfer of antenatal information into over 10% of postnatal records. CONCLUSIONS Current ultrasound techniques for “dating” pregnancy antenatally are better than any of the methods of postnatal assessment. Given the reliability of the antenatal information now available, it is regrettable that so many inaccuracies have been allowed to creep into the routine computation and recording of gestation at birth. Key messages The best techniques for estimating gestation immediately after birth are only half as accurate (95% CI ± 17 days) as estimates based on antenatal ultrasound Such estimates are even less accurate than this in babies of less than 30 weeks gestation Documenting when various reflexes appear can provide a better retrospective estimate of gestation in the preterm baby than methods based on posture, tone and physical appearance The transfer of obstetric information relating to gestation into the neonatal record is frequently faulty; such records were in error by > one week in 15% of preterm babies entered into one recent multi-centre research study, and 92% of the errors involved an under recording of gestation More care should be taken over collecting obstetric information relating to gestation at birth, given its potential value