A new study may offer women one more reason to kick the smoking habit before becoming pregnant: a potentially reduced risk of early miscarriage. Smoking 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 birth weight baby and an increased risk of giving birth prematurely –- and that passive (second-hand) smoke exposure carries similar risks. Cigarette smoke can cause numerous health problems in children that last for years after birth, possibly even an increased risk of childhood leukaemia.

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. If that’s not enough to motivate women to avoid cigarettes, evidence is mounting that exposure to cigarette smoke in pregnancy, even in mothers who don’t smoke, also increases the risk of miscarriage and stillbirth. Some evidence even indicates that heavy smoking by the father (more than 20 cigarettes a day), possibly even before conception, may also increase the risk of miscarriages.

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.

Theoretically, smoking could cause a miscarriage in a number of ways. In the early days of pregnancy when the baby develops so quickly, cigarette smoke may cause genetic damage in the baby. Chromosomal problems are the most common cause of miscarriages, so it’s theoretically possible that heavy exposure to cigarette smoke could be a cause. Smoking could also change the lining of the uterus to make it harder for the fertilized egg to implant.

The latest findings, reported in the journal Human Reproduction, support a connection between smoking and miscarriage.

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. Different studies find different exact figures to indicate the risks of smoking and miscarriage, but regardless of the exact numbers and exact mechanism, quitting smoking or avoiding cigarette smoke is one of the few miscarriage risk factors that you can control. If you smoke and had a past miscarriage, it is impossible to say whether smoking caused your loss or whether it will cause a future miscarriage or stillbirth, but quitting will definitely do you and your future child a favour.

If you are wondering how the father’s smoking could affect miscarriages, the answer may lie in the sperm. A few studies have found that men who smoke heavily tend to have increased incidence of sperm with chromosomal abnormalities, and again, chromosomal abnormalities in the developing baby are the most common cause of miscarriages. The answer could also be that the father’s smoking causes the mother to be around secondhand smoke, which could theoretically cause problems in the mother’s ability to maintain the pregnancy. Future studies will hopefully reveal the exact mechanism behind this link.

Some studies have found an even stronger link between smoking and miscarriages when looking only at miscarriages where the baby was chromosomally normal, however, so the reason why smoking increases miscarriage risks may have nothing to do with chromosmal issues 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 with lower birthweight and can also increase the risk of stillbirth, as well as death in the first year of life.

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.

Related articles:

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2. The causes of miscarriage
3. Treatment for miscarriage
4. The ‘what’ and ‘why’ of miscarriage in early pregnancy.
5. What makes a woman prone to recurrent pregnancy miscarriage?
6. Can I get pregnant just after my period has finished?
7. Working and having a baby

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