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Abortion will no longer increase the risk of preterm birth, scientists say.

British researchers found that the link between abortion and preterm delivery in a subsequent pregnancy has faded away over the last 20 to 30 years.

The findings, published in the journal PLOS Medicine, showed that while abortion was a strong risk factor for subsequent preterm birth in the 1980s, the link has progressively weakened over the past two decades.  In fact, investigators said the link was no longer present among women giving birth from 2000 onwards.

Researchers said the findings are important because current recommendations for women who want to terminate their pregnancies are based on studies before 2000. Researchers said the current guidelines should be revised because the latest analysis clearly shows that there is no link between abortion and the subsequent risk of preterm birth in modern practice.

Researchers from the University of Cambridge examined data from 757,060 live first births (excluding twins) between 1980 and 2008.  Of all the birth, 56,816 women reported one previous termination, 5,790 women reported two previous terminations, and 822 women reported three or more previous terminations.

After accounting for maternal characteristics, researchers found that between 1980 and 1983, there was a 30 percent increase in the risk of spontaneous preterm birth with each previous termination procedure.

However, researchers found that this link progressively weakened, with a 10 percent to 20 percent increase in the risk for preterm births in the 1990s and no link at all from the millennium onwards.

Study authors said the likely explanation for these findings is changes in methods of abortion. Over the period 1992 to 2008, researchers found that the procedure believed most likely to lead to an increased risk of preterm birth (purely surgical abortion without the use of any drugs) decreased from 31 percent in 1992 to 0.4 percent in 2008. What's more, the proportion of medical terminations (procedures that avoided the use of surgery altogether) increased from 18 percent to 68 percent.

Researchers say the use of purely surgical termination may have been responsible for the increased risk of preterm birth.  They believe that the phasing out of purely surgical abortions in Scotland in the 1980s and 1990s may have led to the subsequent disappearance of the established link between previous termination and preterm delivery from 2000 onwards. However, researchers could not directly test whether the two trends were related because they did not have information on the methods of previous termination linked to subsequent birth outcome for individual women.

"We have shown that previous abortion was a risk factor for preterm birth among nulliparous women in Scotland prior to 2000. However, increased use of medical methods of abortion and of cervical pre-treatment prior to surgical abortion has been paralleled by a disappearance in the association," researchers wrote.

"We believe that it is plausible that modernizing methods of termination of pregnancy worldwide may be an effective long-term strategy to reduce future rates of preterm birth," they added.