Interpregnancy interval and gestation at the next birth—is there a racial difference?

The association of short interpregnancy intervals (IPIs) with an increased rate of preterm birth in the next pregnancy has been known for 70 years or more (Eastman Am J Obstet Gynecol 1944;47:445–63). Rawlings et al. in 1995 reported that an IPI of <9 months in black women was significantly associated with a higher preterm birth rate, whereas the effect was only seen in white women when the IPI was <3 months (Rawlings N Engl J Med 1995;332:69–74). In the same journal in 1999, Zhu et al. reported that in a predominantly (90.1%) white/Hispanic population, most of the increased risk associated with a short IPI was in the first 6 months, with only a small fall thereafter, reaching a nadir at 18–23 months, following which it rose again (Zhu et al. N Engl J Med 1999;340:589–94). DeFranco et al. have defined a short IPI as <12 months, because the few pregnancies with an IPI of <6 months ‘did not allow for meaningful analysis of this subgroup’. This means that the majority of the women in their study—white—were at relatively low risk. However, more than a quarter of the women were black, and there are now considerable published data suggesting that black women have physiologically shorter pregnancies with their babies being more mature at birth (e.g. Balchin et al. Obstet Gynecol 2011;117:828–35), so making it more complicated to draw conclusions from data on gestational length in white and black women combined. More of the women with an IPI of <12 months in DeFranco's study were black (25.6%) than in the group with an IPI of ≥18 months (15.9%). Figure 2 in DeFranco's study shows in women with shorter IPIs a higher rate of births at each gestational age <37 weeks but a lower rate of births from 39 weeks onwards. However, the modal gestation is the same for each group, 40 weeks. This is surprising, if as the authors suggest there is ‘a shift of the distribution curve of cumulative births by week of gestation to the left’. A shift of the mode to the left in black women, consistent with more preterm and fewer post-term births, was indeed seen in the Balchin et al. study, and this is what one would expect if there was a physiological total population shift, rather than a pathological increase in preterm births. It may be that in DeFranco's study, the mode is dominated by white women at relatively low risk, while the increase in preterm births (and reduction in post-term births) is most pronounced in black women. Such nuances cannot be determined by multivariate logistic regression as by definition, such modelling incorporates adjustments for the included variables, which in this case, included racial group. See http://www.bjog.org/view/0/editorProfiles.html#psteer.

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