Is the accuracy of prior preterm birth history biased by delivery characteristics?

To assess the sensitivity of birth certificates to preterm birth history and determine whether omissions are randomly or systemically biased. Subjects who experienced a preterm birth followed by a subsequent pregnancy were identified in a regional database. The variable "previous preterm birth" was abstracted from birth certificates of the subsequent pregnancy. Clinical characteristics were compared between subjects who were correctly versus incorrectly coded. 713 subjects were identified, of whom 65.5% were correctly coded in their subsequent pregnancy. Compared to correctly coded patients, patients who were not correctly identified tended to have late and non-recurrent preterm births or deliveries that were secondary to maternal or fetal indications. A recurrence of preterm birth in the subsequent pregnancy was also associated with correct coding. The overall sensitivity of birth certificates to preterm birth history is suboptimal. Omissions are not random, and are associated with obstetrical characteristics from both the current and prior deliveries. As a consequence, resulting associations may be flawed.