Racial variation in the association between gestational age and perinatal mortality: prospective study. BMJ 2007; 334; 833
The average length of gestation for single pregnancies is 40 weeks for white women, 39 weeks for South Asian and 38.8 weeks for black women. If this shorter length of gestation has a physiological basis, the likelihood of perinatal complications may increase earlier in these women.
The aim of this study was to determine if the risks of perinatal mortality and antepartum still birth associated with post term birth, increase earlier during pregnancy in South Asian and black women than in white women ; and to investigate differences in the factors associated with antepartum stillbirth between the racial groups.
This study reviewed patient data from 15 maternity units in Northwest London over 12 years. It analysed outcomes of 197,016 nulliparous women: self reported as white, south Asian or black, who delivered a single baby weighing at least 500g at 24-43 completed weeks gestation.
Of these women, 81% were white women, 13% South Asian and 6% black women. The best estimate of gestational age at delivery was calculated from the first day of the last menstrual period or biparietal diameter measurements taken from fetal ultrasonography recordings made before 24 weeks of gestation.
The paper concluded that perinatal mortality patterns differed significantly with each racial group. Prior to 32 weeks gestation, perinatal mortality was lowest in black women, consistent with the known accelerate pulmonary maturity of fetuses in this racial group. At all gestational ages, perinatal mortality was highest in babies of South Asian women, and from term onwards the increase was earliest and steepest in South Asian women, followed by black women, and finally white women. Allowing for confounders, the risk of antepartum stillbirth was higher for South Asian women whereas black women had higher rates of antepartum stillbirth, intrapartum stillbirth and early neonatal death than white babies. The strongest factor associated with antepartum stillbirth in white women was placental abruption. In South Asian and black women, the strongest factor was having a baby with birth weight below 2000g, possibly as a result of growth restriction. The differences in the incidence of placental abruption and low birth weight partly explained the differences in perinatal mortality.
These complications have led to the practice of offering induction of labour to women after 41 weeks of gestation.
Since South Asian and black women have lower antepartum stillbirth rates at 40 weeks gestation, perhaps increased fetal surveillance and growth monitoring from 40, rather than 41 weeks gestation is justified.