A new study may offer women one more reason to kick the smoking habit before becoming pregnant: a potentially reduced risk of early miscarriage.

In a study of nearly 1,300 Japanese women with a past pregnancy, researchers found that those who smoked heavily early in pregnancy were more than twice as likely as non-smokers to suffer a miscarriage in the first trimester.

There are many reasons for women to quit smoking before becoming pregnant. The habit has been linked to increased risks of stillbirth, preterm delivery and low birthweight.

But studies so far have come to conflicting conclusions as to whether smoking might contribute to miscarriage risk.

These latest findings, reported in the journal Human Reproduction, support a connection.

For the study, researchers led by Dr. Sachiko Baba of Osaka University reviewed the records of 430 women who’d suffered a first-trimester miscarriage. They compared each woman with two others the same age who had given birth that same year.

Overall, the researchers found, women who smoked heavily during pregnancy - at least 20 cigarettes per day - were more than twice as likely as the non-smokers to have a miscarriage.

Seven percent (32) of the 430 women who suffered a miscarriage smoked that amount, versus four percent (36) of the 860 women who delivered a baby.

Previous research has led to estimates that up to eight percent of pregnancies end in miscarriage between six and eight weeks after the woman’s last period, but after 10 weeks that rate drops to two percent.

Most miscarriages happen in the first trimester and experts believe that the majority of those are caused by random genetic abnormalities that cannot be prevented. However, certain lifestyle habits have been linked to a relatively increased risk of miscarriage - including heavy drinking, drug use and, in some studies, smoking.

The current findings do not prove that smoking, itself, was the reason for the increased miscarriage risk seen in the study group. But the researchers were able to account for several other factors, including the women’s reported drinking habits and histories of past miscarriages. And the smoking-miscarriage link remained.

Baba’s team also found that women who worked during the first trimester were 65 percent more likely to have a miscarriage than those who did not work outside the home.

Of women who suffered a miscarriage, one-third said they’d worked outside the home, compared with 19 percent of those who gave birth.

It’s not clear what to make of that finding, according to the researchers. One problem is that they lacked any employment information on a large portion of the study group - 35 percent.

More importantly, they could not divide the women into job types.

Some studies, Baba’s team notes, have linked certain jobs with high physical demands or chemical exposures to miscarriage risk. One recent study, for instance, found that women exposed to anesthesia fumes in veterinary centers had a higher-than-average miscarriage risk.

However, the researchers write, they know of no studies outside of Japan that have linked employment, as a whole, to an increased miscarriage risk.

More research is needed, they say, to explain the finding.

According to the American College of Obstetricians and Gynecologists, there is no proof that working during pregnancy raises a woman’s miscarriage risk.

SOURCE: Human Reproduction, online December 14, 2010.