US trends in abortion and preterm birth.

BACKGROUND

A recent large meta-analysis concluded that prior surgical abortion was an independent risk factor for spontaneous preterm birth (PTB), while they found no significant correlation between PTB and medical abortion.

OBJECTIVE

To evaluate the potential impact of changes in US abortion rates and practices on US incidence of PTB rate.

STUDY DESIGN

This was an epidemiologic analysis of legal abortion and PTB data in the USA from 2003 to 2012. Birth data (annual total birth, annual number and incidence of PTB, defined as PTB <37 weeks) are from National Vital Statistics Reports from the National Center for Health Statistics, Center of Disease Control and Prevention (CDC). Abortion data were collected using Abortion Surveillance provided by the CDC. Abortion incidence was reported overall, and by type: surgical, medical method and procedures reported as "other" such as intrauterine instillation and hysterectomy/hysterotomy. To test for the trend of abortion and of PTB over time, we used the chi-squared test for trend. The primary outcome of our study was the correlation trend analysis between abortion rate and PTB rate. Pearson correlation test was used. A two-tailed p value of 0.05 or less was considered significant.

RESULTS

From 2003 to 2012 there were 41 206 315 births in USA, of which 5 042 982 (12.2%) were <37 weeks. The PTB rate declined significantly from 12.3% in 2003 to 11.5% in 2012 (p value test for trend <.04). Out of the 6 122 649 legal abortions, reported by type of procedure, performed from 2003 to 2012 in USA, 5 132 789 were surgical abortion (82.8%) and 860 288 (14.0%) were medical. Chi-squared test for trend showed that the rate of surgical abortion significantly decreased from 88.9 to 78.0% (p < .01) while the rate of medical abortion significantly increased from 7.9 to 21.9% (p < .01) from 2003 to 2012, respectively. The rate of PTB was correlated with the rate of medical abortion (p = .01) and of surgical abortion (p = .02) over time. The higher the surgical abortion rate, the higher the incidence of PTB (Pearson correlation 0.712); the higher the medical abortion rate, the lower the incidence of PTB (Pearson correlation -0.731).

CONCLUSION

Recent changes in abortion practices may be associated with the current decrease in US incidence of PTB. Further study on the effect of surgical versus medical abortion is warranted regarding a possible effect on the incidence of PTB.