WHY DO ABORIGINAL NEWBORNS WEIGH LESS - GESTATIONAL-AGE AT DELIVERY - ESTIMATION, DISTRIBUTION AND DETERMINANTS

We reviewed birth records pertaining to 1,301 Western Australian singleton Aboriginal pregnancies dated before 24 weeks by ultrasound fetometry using Caucasian standards. We compared i) ultrasound- and LMP-derived estimates of age at delivery for 323 pregnancies with available last menstrual period (LMP) dates and ii) distributions of ultrasound-estimated age at delivery for full-blood with part-blood Aboriginal neonates and with statutorily collected estimates for Caucasian neonates. We investigated determinants of age at delivery for Aboriginal neonates. Ultrasound-derived estimates of gestational age for Aboriginal pregnancies were statistically but not clinically significantly shorter than estimates derived from LMP dates claimed to be certain: mean difference = 0.4 weeks (95% CI:0.09-0.71). Aboriginal gestations were shorter than Caucasians' by 2 weeks (median) or 1 week (mode). Some shortening of gestation was accounted for by antepartum haemorrhage, congenital anomaly, history of syphilis, diabetes, hypertensive disease and inductions before 40 weeks without recorded medical complication but not by full-blood (relative to part-blood) Aboriginality. We conclude that Caucasian fetometry standards gave gestational estimates acceptably close to those derived from certain menstrual dates for dating scan before 24 weeks. Aboriginal gestations tended to be shorter than Caucasian gestations but since factors recorded in the medical record accounted for little of this difference, socioeconomic factors may be partly responsible.