Placenta previa is defined as the implantation of the placenta in the lower segment of the uterus. It occurs in 3 out of every 1000 pregnancies Findeklee and Costa, 2015. The risk factors for placenta previa are smoking, previous cesarean sections, advanced maternal age, multiparity and conception by in vitro fertilization (IVF) Shobeiri et al., 2017. Abortions have been proposed to be associated with fetal pathology, congenital abnormality, low birth weight and preterm labor in subsequent pregnancies Kashanian et al., 2006.

There is controversy regarding the role of prior abortion on placenta previa in subsequent pregnancies. Some studies have reported an increased risk of placenta previa following abortions Chelmow et al., 1996Eniola et al., 2002Handler et al., 1994Newton et al., 1984ROSE and CHAPMAN, 1986Thom et al., 1992, while others saw no correlation Bakshi et al., 2015Kashanian et al., 2006Latif et al., 2015Usta et al., 2005.

Two previous meta-analyses have shown a positive association between previous abortion(s) and placenta previa in subsequent pregnancies Ananth et al., 1997aFaiz and Ananth, 2003. However, such studies have encountered limitations, such as the limited number of primary databases. Thus, in this study, we performed meta-analysis, based on a larger number of subjects and databases to screen, to address the association of previous abortions and placenta previa. Thus, we conducted an updated and comprehensive meta-analysis of the placenta previa after previous abortion(s).

The present meta-analysis study was conducted based on the PRISMA guidelines.

The meta-analysis described herein and based on observational studies show that there is an association between prior abortion and placenta previa. Our results suggest that prior abortion is a risk factor for placenta previa. It has been previously reported that placenta previa is correlated with maternal and fetal complication, such as antenatal and post-partum hemorrhage, preterm delivery, intrauterine growth restriction, malpresentation and poor neonatal outcomes Rombauts et al., 2014.

In a meta-analysis report published by Ananth et al. in 1997 Ananth et al., 1997b, the authors showed that based on OR estimates from all the studies they evaluated, there was a significant increase in the risk of placenta previa after prior spontaneous abortions (1.7; 95% CI: 1.5, 2.0) and after prior induced abortions (1.5; 95% CI: 1.3, 1.7). However, this meta-analysis was limited to eight studies and they searched only the Medline database.

In another meta-analysis, conducted in 2003 by Faiz et al. and which evaluated all articles up to year 2000, the authors showed that prior abortion increased the risk of placenta previa. Based on their results, there was significant association in the increased risk of placenta previa after prior spontaneous abortions (2.0; 95% CI: 1.7, 2.3) and after prior induced abortions (1.5; 95% CI: 1.3, 1.9) Faiz and Ananth, 2003. For their analysis, the authors searched only the Medline database and the meta-analysis included 9 observational studies.

The mechanisms involved in the association of prior abortions and placenta previa is unknown. The damage and scarring to myometrium and endometrium of the uterus during spontaneous and induced abortions may influence the low implantation of placenta in the uterus in subsequent pregnancies Faiz and Ananth, 2003.

The meta-analysis described herein had two limitations. While some studies report only the unadjusted form of OR, we tried to use the adjusted form to control for risk factors which may have impacted the studies included in this meta-analysis. However, doing so might introduce information bias in our results. Also, in the present study, we attempted to identify all published studies. However, in spite of our efforts, we could not find two studies that might have reported data on placenta previa. Despite these limitations, the present meta-analysis study was drawn from a large sample size; the 20 studies should efficiently estimate the association between prior abortion and risk of placenta previa. Our results indicate, based on odds ratio reports in epidemiological studies, that prior abortion (spontaneous and induced) can increase the risk of placenta previa.

We showed based on our present meta-analysis of observational studies that prior spontaneous and induced abortions can increase the risk of placenta previa. Therefore, prior abortion is a risk factor for placenta previa.

EJ and MK designed the study and processed the data. MK and EJ performed the statistical analysis. EJ and MK interpreted the results and wrote the first draft. Two authors read and approved the final manuscript.