There is some concern about the effects of induced abortion on subsequent pregnancy outcomes, in part because abortion usually is induced early or about halfway through a woman’s reproductive life. Recent studies have shown an excessive risk of preterm delivery. This study examined the risk of very preterm birth, at 22 to 32 weeks gestation, as related to previous induced abortion. A case–control study enrolled 1843 very preterm live-born singleton infants at less than 33 weeks gestation, 276 infants born at 33 to 34 weeks gestation (moderately premature), and 618 unmatched full-term control women who delivered at 39 to 40 weeks duration. Mothers of preterm infants were younger than those with full-term infants, had less education, and were likelier to be living alone, to be unemployed, and to smoke.
A history of induced abortion correlated with an increased risk of very preterm birth (odds ratio, 1.6; 95% confidence interval, 1.2–2.1). There was little change when controlling for maternal characteristics or without adjusting for a history of preterm deliveries. In addition, the association remained the same when women with previous preterm delivery were excluded. The risk tended to increase with the number of previous induced abortions. The adjusted risk of preterm delivery associated with induced abortions tended to be highest for extremely preterm deliveries. The major complications leading to very preterm birth were premature rupture of membranes and idiopathic spontaneous preterm labor, and these occurred more often in connection with extremely preterm birth. Hypertension and fetal growth restriction were more common when infants were born at 28 to 32 weeks gestation. An association between previous induced abortion and very preterm delivery related to fetal growth restriction was apparent in infants born at 28 to 32 weeks gestation.
This study shows that a history of induced abortion increases the risk of very preterm birth, particularly extremely preterm deliveries. It appears that both infectious and mechanical mechanisms may be involved.