How Smoking During Pregnancy Causes Miscarriage

Pregnant Woman Smoking
IAN HOOTON

Smoking—especially during pregnancy—is a risky move. For years, doctors have known that women who smoke while pregnant have almost double the risk of having a low-birthweight baby and an increased risk of giving birth prematurely. Even exposure to secondhand smoke carries risks. Cigarette smoke can cause numerous health problems in children that last for years after birth. 

If that’s not enough to motivate pregnant women to kick the habit themselves or steer clear of others who light up, the evidence is mounting that exposure to secondhand cigarette smoke in pregnancy—even in mothers who don’t smoke—also increases the risk of miscarriage and stillbirth. There also is some evidence that when a dad-to-be is a heavy smoker (more than 20 cigarettes a day) his habit could increase his partner's risk of miscarriage.

How Could Smoking Cause a Miscarriage?

During the first few days after conception, when the fetus is developing rapidly, it's highly susceptible to genetic damage caused by cigarette smoke. And because chromosomal problems are the most common cause of miscarriages, it’s possible heavy exposure to cigarette smoke could play a role. Smoking also can affect the lining of the uterus, making it difficult for a fertilized egg to implant.

As for the possible role of dads who smoke in the risk of miscarriage, a few studies have found that men who smoke heavily tend to have increased incidence of sperm with chromosomal abnormalities. And of course, if a father-to-be lights up around his pregnant partner, he's exposing her to secondhand smoke.

Other studies have found an even stronger link between smoking and miscarriages when looking at only miscarriages in which the baby had normal chromosomes. So the reason why smoking increases miscarriage risk may have nothing to do with chromosomal problems and could have more to do with something else, such as the placenta having a diminished capacity to transport oxygen and nutrients to the fetus.

Research indicates that later in pregnancy, smoking does appear to decrease the placenta’s ability to deliver nutrients to the developing baby. In addition to potentially causing miscarriages, this can cause babies to be born at a low birth weight and also can increase the risk of stillbirth, as well as death in the first year of life.

As yet there's no agreement about the amount of smoking likely to increase the risk of miscarriage (an occasional cigarette versus a pack a day, for example). However, since kicking the habit is one of the few risk factors parents-to-be can control in order to help prevent pregnancy loss, it makes sense to do it—not only for the sake of your baby's health but also for yours.

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