Despite the potential risks to their baby's health, roughly one in 10 women smoke in the three months before getting pregnant, U.S. health officials reported.
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And only one-quarter of those women quit before they become pregnant, the researchers said.
"Smoking during pregnancy is double trouble. There is a mountain of research that it does affect the unborn child," said lead researcher Sally Curtin, a statistician at the U.S. Centers for Disease Control and Prevention's National Center for Health Statistics.
Dr. Edward McCabe, senior vice president and chief medical officer at the March of Dimes, said, "We know that smoking is a problem for pregnancy, and we continue to see many women smoking."
Smoking leads to premature birth and low birth weight infants, which increases the odds for mental and developmental problems that can affect someone for their whole life, McCabe said.
Premature birth is also a risk factor for cerebral palsy, lung disease and premature death, he added.
McCabe said that women need to make their own decisions about smoking, but they should plan their pregnancies and get their bodies in the best health possible beforehand.
Curtin believes that smoking among pregnant women is under-reported and many more smoke than admit to it.
"But even 8.4 percent who smoked at any time during pregnancy is a lot of women. We have 3.8 million births - that's about 300,000 babies each year."
Using birth certificate data from 2014, the researchers were able to find out the average number of cigarettes smoked before and during pregnancy, along with how many women quit before and during pregnancy.
According to the report, smoking was most prevalent among women aged 20 to 24 (13 percent). In addition, American Indian and Alaska Native women were most likely to be smokers (18 percent).
Smoking during pregnancy in most states averaged about 10 percent, ranging from almost 2 percent in California to about 27 percent in West Virginia, Curtin said. ■