Secondhand smoke during pregnancy alters children's behavior.

Mar 25, 2013

Liu, J, PWL Leung, L McCauley, Y Ai and J Pinto-Martin. 2012. Mother's environmental tobacco smoke exposure during pregnancy and externalizing behavior problems in children. NeuroToxicology http://dx.doi.org/10.1016/j.neuro.2012.11.005.

Synopsis by Glenys Webster

Mothers who breathe secondhand smoke are twice as likely to have children with attention and aggression problems, a study from China reports. While it is known that smoking during pregnancy can affect a child's behavior, this is one of the first studies to show a link to secondhand smoke. Women can be exposed at home or at work when they breathe the smoke from a burning cigarette or smoke exhaled by a smoker.

Children exposed to secondhand smoke in the womb are more likely to have behavior problems when 5 to 6 years old, reports a study from China.

Kids with pre-birth exposures were twice as likely to have attention and aggression problems – together called “externalizing" behaviors – compared to non-exposed kids. Prebirth exposures are of particular concern because of the negative effects on the developing fetus.

This is one of the first times that passive – rather than active – smoking during pregnancy has been linked to child behavior. The results from this study are consistent with prior studies from the United States and Taiwan.

Secondhand smoke comes from the end of a burning cigarette or is exhaled by a smoker. U.S. health and regulatory agencies consider it a human carcinogen. Exposure to secondhand smoke also increases the risk of sudden infant death syndrome and affects children's respiratory and lung health.

While fewer than 2 percent of women in China are smokers, more than 40 percent are exposed to secondhand smoke during pregnancy. The problem is not isolated to that country. According to a report by the Centers for Disease Control, women of childbearing age exposed to secondhand smoke at work and at home is a worldwide health problem.

The number of women smokers is on the rise around the world, too. By 2025, 1 in 5 women worldwide are expected to smoke.

Women who are active smokers during pregnancy are more likely to have children with behavior problems such as ADHD, conduct problems and aggression. Kids born to smoking mothers also have increased criminal behavior in adolescence and adulthood.

However, less is known about whether secondhand smoke exposure during pregnancy also affects child behavior.

To find out, researchers assessed exposure to secondhand smoke in 646 mothers and children from the town of Jintan in southeastern China who were part of the larger 2005 China Jintan Child Cohort study. When the children were between 5 and 6 years old, mothers filled out questionnaires about their exposure at home, work and elsewhere to secondhand tobacco smoke during pregnancy. Another standard questionnaire was used to assess their children’s behavior and emotional problems.

Child behaviors were scored and grouped into internalizing behaviors – including the sum of emotional reaction, anxiety and depression scores, among others – and externalizing behaviors – the sum of attention problems and aggression scores. Total behaviors included a child’s total score on all of these measures.

More than one-third of mothers (37 percent) reported exposure to secondhand smoke during pregnancy. Kids with higher prebirth exposures were twice as likely to have externalizing behavior problems than non-exposed kids. No links were found between prebirth exposures to secondhand smoke and internalizing or total behavior problems.

The results take into account the child’s sex and school area, the parents’ education, occupation and psychological problems, the mother’s age during pregnancy, and the father’s current smoking status.

Tobacco smoke contains thousands of chemicals, many of which are known to affect brain development. Individual chemicals are known to increase brain cell death, alter brain structure and decrease communication between neurons. Nicotine – the main addictive substance in tobacco – and metals, such as mercury, lead and cadmium, are all potent neurotoxicants.

A possible explanation for the findings is that exposures to secondhand smoke may reduce the flow of blood and oxygen to the fetus, which may alter brain growth and development during pregnancy and thus affect behavior.

An important limitation is that mothers reported from memory their secondhand smoke exposures during their pregnancies 5 to 6 years earlier. The reported exposures may not be accurate. Also, because of the social stigma against women smoking in this area of China, the researchers were not able to determine if the women were themselves active smokers during their pregnancies.

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