Researchers at McGill University's Department of Obstetrics and Gynecology reviewed the records of nearly 18,000 women who gave birth from 2001-2006. They found that women who reported having a previous abortion were more likely to have given birth prematurely.
The conclusion of their study's abstract reads:
Our study showed a significant increase in the risk of preterm delivery in women with a history of previous induced abortion. This association was stronger with decreasing gestational age.
ABSTRACT OF STUDY (PDF)