OBJECTIVES: To investigate the effect of excluding cases with unrecorded best estimate of gestational age at birth on pregnancy outcome reporting and to determine the reasons for unrecorded gestational age data. DESIGN: Prospective study. SETTING: Fifteen maternity units in North West London. POPULATION: 497,105 women who booked for antenatal care from 1988 to 1998. METHOD: Multiple logistic regression analysis. MAIN OUTCOME MEASURES: Preterm birth rate of, and the factors associated with, cases with unrecorded best estimate of gestational age at birth. RESULTS: Of the 53,981 cases with an unrecorded best estimate of gestational age at birth, by using additional data, it was possible to compute a new best estimate of gestational age in 80%. In this latter group, the preterm birth rate was 42% (95% CI 41.5-42.6). The corrected, overall preterm birth rate in North West London (9.8%, 9.7-9.9) was higher than the original estimate (7.6%, 7.5-7.7), which included only cases with recorded data on gestational age at birth. The most significant factors associated with an unrecorded gestational age were no ultrasound scan (OR 49, P < 0.001), and preterm birth <31 weeks (OR 30, P < 0.001). CONCLUSIONS: The incidence of preterm birth are likely to be under-reported in studies where only cases with readily available gestational age data are included. In routinely collected maternity data, human omission is an important contributing factor for an unrecorded best estimate of gestational age at birth. This is associated with the urgent transfer of babies to the neonatal intensive care unit.