British researchers have reported another side effect of smoking during pregnancy. They found that smoking in pregnancy hurts both the mother's and the baby's thyroid functioning.

Cigarette smoke has previously been linked to smaller babies, to be predisposing babies to sudden infant death syndrome, and to affect the rates of cleft lips, heart defects and other problems.

The researchers studied the influence of cigarette smoking on thyroid functioning of two groups of women at different stages of pregnancy, in the first and in the third trimester.

In both groups, there has been seen that smoking was associated with changes in the mothers' thyroid hormone levels.

Good thyroid function is the key to maintaining a healthy pregnancy. If a woman suffers from thyroid imbalance during her pregnancy, it can affect her metabolism and rise the chances of miscarriage, premature birth, low birth weight and impaired brain development.

The researchers 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. However, in women who quit smoking in pregnancy, thyroid hormone levels were comparable to levels found in nonsmokers, which suggests that the thyroid changes can quickly clear up.