Women who have had an abortion are no longer at increased risk for preterm birth in subsequent pregnancies, and it may be time to adjust counseling guidelines accordingly, a study has shown.
In a cohort study among 416,301 nulliparous Scottish women with a first singleton birth between 1992 and 2008, those who had had a previous induced abortion had a significant 12% increase in the adjusted odds of spontaneous preterm birth, lead investigator Clare Oliver-Williams reported in a poster session at the Pregnancy Meeting, the annual meeting of the Society for Maternal-Fetal Medicine.
Susan London/IMNG Medical Media
Ms. Clare Oliver-Williams
However, more-detailed analyses stratified by time period showed that the association weakened over time and disappeared as of about 2000.
This trend coincided with a shift toward the use of abortion practices less likely to injure the cervix: a sharp decrease to almost 0 in surgical abortions performed without cervical pretreatment and a sharp increase in medical abortions.
"We think the reason why there has been a loss of association [between previous abortion and preterm birth] is because of this change in practice: There are no longer surgical abortions without cervical pretreatment, and that was driving the association all along," Ms. Oliver-Williams said in an interview.
"If women have chosen to have an abortion previously and then go on to have an intended pregnancy, this should be reassuring to them," she said. And the findings have related implications for preabortion counseling.
Current U.K. guidelines recommend that women be advised before the procedure about a subsequent increase in the risk of preterm birth, she noted. "That might not be the most appropriate thing to tell women anymore given that the majority of abortions [now] occur through medical means or surgical means with cervical pretreatment."
Cervical pretreatment in the United Kingdom usually involves the use of a prostaglandin, and practices may differ across countries, said Ms. Oliver-Williams, a PhD candidate in the department of public health and primary care at the University of Cambridge (England). But it is reasonable to assume that other countries are also getting away from more-injurious procedures.
"The theory holds up that mechanical dilation would damage the cervix and lead to spontaneous preterm birth" in a later pregnancy, she said. "So, in a cautious way, I would suggest that the association would still exist in other countries if there was a decrease in these surgical abortions without cervical pretreatment."
Nulliparous women were chosen for analysis in part because there are fewer complicating factors for this group, such as the timing of abortions relative to births, according to Ms. Oliver-Williams. But the findings would likely be similar for multiparous women, too. "I can’t imagine why multiparity would have any impact," she said.
The main results showed that women with a previous induced abortion had significantly higher odds of spontaneous preterm birth after other factors were considered (odds ratio, 1.12), reported Ms. Oliver-Williams.
There was a dose-response relationship between the number of previous abortions and risk: Women who had had one, two, and three previous abortions had 7%, 24%, and 37% higher odds, respectively, of spontaneous preterm birth (P less than .001 for trend).
In time period–stratified analysis, the odds were elevated for women giving birth during 1992-1995 and during 1996-1999. But they were no longer so thereafter.
During the same overall study period, the proportion of all abortions that were surgical and performed without cervical pretreatment fell sharply (from 31% to less than 1%), and the proportion that were performed with medication rose sharply (from 18% to 68%).
In additional study findings, previous abortion was not associated with a significant increase in the risk of various other adverse outcomes: antepartum stillbirth, intrapartum stillbirth, neonatal death, small-for-gestational-age birth, induced preterm birth due to any reason, or induced preterm birth specifically due to preeclampsia.
Ms. Oliver-Williams disclosed no relevant financial conflicts.