Even when mothers quit smoking early in pregnancy babies still at risk of smaller body size

New European research has found that although mothers who quit smoking during the first trimester of pregnancy are reducing the negative risk that smoking poses to their unborn child, it still isn't enough to protect babies from being born shorter and with smaller brains.

Carried out by the University of Eastern Finland, the new study looked at 1.38 million mother-child pairs in Finland and recorded the body size and body proportions of the newborn babies when the mothers had smoked only during the first trimester and also throughout the whole pregnancy.

The findings, published in BMJ Open, showed that smoking while pregnant was linked with a stronger reduction in body length and head circumference than a reduction in birth weight, which leads to changed body proportions.

Although the effect of smoking on body proportions was slightly lower when the mothers quit smoking during the first trimester, the newborns who were exposed to maternal smoking during this short time still showed growth restriction in all three measurements of body size -- birth weight, body length and head circumference -- and showed abnormal body proportions.

The researchers say this shows that early pregnancy is still a "sensitive exposure window."

In addition, the researchers also found that the potential to repair fetal damage caused in early pregnancy was limited.

"Smoking during pregnancy is relatively common. In this study, 84.5 percent were non-smokers and 3.5 percent quit smoking during the first trimester, but 12% continued to smoke after the first trimester," said researcher Isabell Rumrich.

"The most important finding of our study is that although quitting smoking in the first trimester reduces the risk of low birth weight, brain size and body length in relation to body weight seem not to catch up," added Rumrich. "This stresses the importance of quitting smoking already before pregnancy, since even smoking only during early pregnancy can have devastating effects on the long-term health of the unborn child."

The team also point out that tobacco smoke contains thousands of chemicals which can cross the placenta, potentially affecting the development of organs, including the brain, and the oxygen supply of an unborn child.