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A number of investigations into a possible association between induced abortion and subsequent preterm birth have occurred over the past 30 years.  Many of the early studies found no association. 

Then, as larger studies with improving methodology were performed, there were some that began to find an increased risk of preterm birth following an induced abortion.  Arguments could be made that the methodology of some of these studies was less than desired, and that confounding factors could account for these findings–allowing those who didn’t want to believe there was an association to continue in their disbelief.

But the studies finding an association continued to accumulate, and there were not many, if any, that were finding no association.

In the last couple of years two important studies in this area have emerged that deserve attention. 

The first, published in 2004, was a study that evaluated preterm birth and its relationship to prior induced abortion from the EUROPOP survey dataset, a case-control study from many countries across Europe.(1) This dataset was collected in 1994-1997 and has been used for a number of studies looking at factors associated with preterm birth. 

For this particular study, data from ten different European countries was evaluated, including countries from Western, Northern and Eastern Europe. Among women who had one or more induced abortions there was a subsequent adjusted odds ratio of 1.5 for very preterm births (22-32 weeks EGA).

They also found that the adjusted odds ratio increased to 1.8 when women with two or more prior abortions were analyzed separately.  The adjustments took into account most all of the major known risk factors for preterm birth.

The second study, published in 2005, took a page from the first study, and used the EPIPAGE case-control dataset that had been collected at various maternity centers in France in 1997.(2)  Data from France had not been included in the prior study. 

Again, in this study, adjustments were made for major known risk factors for preterm birth. Among women who had one or more abortions there was a subsequent adjusted odds ratio of 1.5 for very preterm births (22-32 weeks EGA).

When data was analyzed separately for women who had two or more abortions the adjusted odds ratio for preterm birth increased to 2.6. In this particular study they also looked at extremely preterm deliveries (22-27 weeks) and found an adjusted odds ratio of 1.7 for those who had at least one prior abortion.

The abortions obtained by women in these studies were virtually all by dilatation and curettage or suction curettage and therefore do not give us any information about abortions obtained by medical methods.

It should be noted that there were a number of authors who took part in both studies. This imparts the advantage of two different datasets being evaluated in a similar fashion, leading to a uniformity of analysis that would be difficult to achieve with two entirely separate groups of researchers. On the other hand, any biases that might have been introduced by the researchers are likely to be present in both studies. It is interesting to note how closely the findings of the two studies correlate.

Both studies were unmatched case-control studies. Given the nature of the subject matter this is probably the best study methodology we will see for this particular issue, although a matched case-control study might be somewhat more reliable. 

The EUROPOP study suffers from the weakness of the abortion history being obtained from the mothers after they had delivered. The authors appropriately note that this could introduce a recall bias into the study, with mothers who had just delivered preterm perhaps as a group being either more or less likely to report a prior abortion to a researcher.

However, in the EPIPAGE study, the data regarding prior abortions was obtained at the beginning of pregnancy, and any selective reporting of abortions would likely be spread more evenly between those who subsequently delivered preterm and those who delivered at term.  In fact, looking at prior studies that have evaluated patient characteristics related to underreporting of abortions suggests that, if anything, underreporting of abortion in the EPIPAGE study may have led to an underestimation of risk for preterm delivery following abortion. What is notable is that both studies came out with very similar results, despite these potential difficulties.

It is important to keep these results in a proper perspective.  Smoking has also been associated with preterm delivery.

The EPIPAGE dataset has been evaluated for the effect of smoking on preterm birth, while adjusting for other risk factors.(3) The result is that the adjusted odds ratio for preterm birth (27-32 weeks EGA) is 1.7 for smokers. Therefore the association between smoking and preterm birth approximates the association between prior induced abortion and preterm birth.

The bottom line is this:  The most recent and best studies looking at the issue of induced abortion and subsequent risk of preterm birth confirm the findings of many prior studies which may have been less rigorous in study design. 

Induced abortion IS associated with subsequent preterm birth, it is even more strongly associated with extremely preterm births, and having two or more abortions further increases these risks.

These studies have made it increasingly impossible to ignore the association of preterm birth with prior induced abortion.

One may quibble about study methodology and hope that a future study will find something different, but for the time being, these results represent the state of the art.

References:< br /> Ancel P, et al. History of induced abortion as a risk factor for preterm birth in European countries: results of the EUROPOP survey. Hum Reprod 2004;19:734-40.

Moreau C, et al. Previous induced abortions and the risk of very preterm delivery: results of the EPIPAGE study. Br J Obstet Gynaecol 2005;112:430-437.

Burguet A. The Complex Relationship Between Smoking in Pregnancy and Very Preterm Delivery. Results of the EPIPAGE Study. Br J Obstet Gynaecol 2004;111:258-265.
AAPLOG, April 2006