Background Accurate assessment of gestational age at birth is critical to identify neonates at high risk of morbidity. Postnatal techniques are commonly used in resource poor settings but may be difficult to apply and have not been well validated in community studies. The aim of this study was to evaluate postnatal assessment of gestational age using the external criteria of the Ballard examination against first/early second trimester ultrasound and date of last menstrual period, in rural Africa.

Method The precision of gestational age estimates by the external Ballard examination was compared to those derived from first and early second trimester ultrasound examination and date of the last menstrual period (n=82) in a sample of women from Kiang West, The Gambia.

Results The external Ballard examination tended to underestimate gestational age by 15.9 days (SD 10.9 days) when compared to gestational age derived from ultrasound and underestimate by 15.4 days (SD 23.1 days) when compared to gestational age derived from date of last menstrual period.

Conclusion Postnatal assessment of gestational age by external Ballard examination performed poorly when compared to ultrasound and last menstrual period in this rural community setting. No reliable gestational age could be derived from its estimate and it failed to detect a significant proportion of high risk infants. The development of a new or adapted postnatal gestational age assessment method specifically for use by health workers in rural Africa may be required.