Children whose mothers smoked during pregnancy may have an increased risk of asthma - even if they were not exposed to secondhand smoke after birth, a large study of European children suggests.

Many studies have found that secondhand smoke may worsen kids' asthma symptoms, or possibly raise their risk of developing the lung disease in the first place.

But it's been less clear if smoking during pregnancy is linked to asthma. Most studies have not been able to tease out the possible effects from those of secondhand smoke after birth.

The new study, however, had a large enough group of kids who were exposed to smoking in the womb, but not after birth, according to the researchers.

And it found that those children were two-thirds more likely to have asthma by age six, versus kids whose moms did not smoke during pregnancy. Even smoking during the first trimester alone was linked to higher asthma risk.

The findings cannot prove that prenatal smoking was the cause.

But there are already plenty of reasons for women who are planning to have a baby to quit smoking, said Anna Bergstrom, a researcher at the Karolinska Institute in Stockholm who worked on the study.

Smoking during pregnancy is linked to increased risks of miscarriage, low birth weight, certain birth defects and other pregnancy complications.

"I think that our study provides yet another good reason to quit smoking," Bergstrom said in an email.

The findings, published online in the American Journal of Respiratory and Critical Care Medicine, come from data on 21,600 European children.

There were 735 children whose mothers said they'd smoked during pregnancy but not after giving birth.

Just under seven percent of all children in the study had been diagnosed with asthma by the time they were four to six years old. And the risk, Bergstrom's team found, was higher when mothers had smoked during pregnancy.

Those children were 65 percent more likely to develop asthma, when factors like birthweight and both parents' own history of asthma were taken into account.

These types of studies point to correlations, and cannot prove cause-and-effect.

But Bergstrom said it is "biologically plausible" that prenatal smoking raises a child's future asthma risk. Exactly how is not clear, but chemicals in tobacco smoke may affect early lung development.

And if there are direct effects, they may happen early in pregnancy, the current findings suggest.

Of the mothers who smoked only during pregnancy, most quit during the second or third trimester. But first-trimester smoking alone was linked to a doubling in a child's risk of asthma.

"For many reasons - not only thinking about asthma in the offspring - it is a good idea to quit smoking when planning a pregnancy, or as early as possible in a pregnancy," Bergstrom said.

Some pregnant women may be able to quit smoking with behavioral counseling. In some cases, a doctor may prescribe nicotine replacement therapy or other medication.