THURSDAY, March 26 (HealthDay News) -- Pregnant women who quit smoking before the 15th week of pregnancy reduce their risk of premature birth and having small babies to that of nonsmoking women, a new study finds.

It's known that smoking during pregnancy increases the risk of miscarriage, ectopic pregnancy, premature birth, small babies, stillbirth and neonatal death, but no study until now has determined whether stopping smoking in early pregnancy reduces the risks of small babies and premature births, the study authors said.

"Pregnant women who smoke should be encouraged and assisted to become smoke-free early in pregnancy," said lead researcher Dr. Lesley McCowan, an associate professor of obstetrics and gynecology at the University of Auckland in New Zealand.

Women who don't quit smoking by 15 weeks are three times more likely to give birth prematurely and twice as likely to have smaller babies, compared to women who stopped smoking, McCowan said.

The findings were published in the March 27 online issue of BMJ.

For the study, McCowan's team collected data on 2,504 pregnant women. Eighty percent did not smoke, 10 percent had quit smoking and 10 percent were current smokers.

There was no difference in the rate of spontaneous premature birth between women who did not smoke and those who had stopped by week 15 (4 percent vs. 4 percent). The same was true for having smaller babies (10 percent vs. 10 percent), the researchers found.

However, women who continued to smoke had higher rates of spontaneous preterm birth than woman who quit (10 percent vs. 4 percent) and higher rates of smaller babies (17 percent vs. 10 percent).

The study also found that women who stopped smoking weren't more stressed than women who continued to smoke, McCowan noted.

"Health professionals who care for pregnant women need to ask about smoking, advise about the importance of stopping, and, where possible, refer for extra support early in pregnancy to assist women to become smoke-free," she advised.

Dr. Richard Frieder, an associate clinical professor of obstetrics and gynecology at the University of California, Los Angeles David Geffen School of Medicine, said the study authors didn't explain whether this difference in low birth weight and gestational age at delivery actually translates into a measurable difference in newborn health.

"We assume this to be true, but there was only about a half pound difference in birth weight and six days difference in gestational age. These numbers are not very impressive that one would think a big difference in neonatal health would be achieved. Still, we should assume that a half pound and six days more of gestation is better and that we should strive to help women stop smoking," he said.

There are other big issues at play when it comes to babies, women and cigarette smoke, Frieder added.

"Babies that live in 'smoking homes' have a much higher risk of respiratory ailments, such as asthma and pneumonia and SIDS. In addition, women are more susceptible to the cancer-causing effects of cigarette smoke than men. The tobacco industry specifically targets women, despite this well-known fact," he said.

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