WASHINGTON (Reuters) - Here's another thing that smoking while pregnant can do -- it can damage both the mother's and the baby's thyroid function, British researchers reported on Tuesday.

Cigarette smoke has been shown to cause babies to be born smaller, to make newborns more likely to die of sudden infant death syndrome, and even to affect the rates of cleft lips, heart defects and other problems.

Bijay Vaidya of Royal Devon and Exeter Hospital and colleagues found smoking can also affect the thyroids of both mothers and babies.

"We studied the influence of cigarette smoking on thyroid function of two groups of women at different stages of pregnancy -- one in the first trimester and the other in the third trimester," Vaidya said in a statement.

"In both groups we found that smoking during pregnancy is associated with changes in the mothers' thyroid hormone levels."

Good thyroid function is key to maintaining a pregnancy, and some pregnant women suffer from thyroid imbalances. This, in turn, affects metabolism and the risk of miscarriage, premature birth, low birth weight and impaired brain development.

Writing in the Journal of Clinical Endocrinology & Metabolism, Vaidya and colleagues said they measured thyroid hormone levels in the umbilical cords of babies born to smoking mothers and found that smoking-related changes in thyroid function also affected the newborn.

But among women who quit while pregnant, thyroid hormone levels were comparable to levels found in nonsmokers, which Vaidya said suggests the thyroid changes can quickly clear up.

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