Babies born to mothers whose bodies don't produce enough of the thyroid hormone thyroxine are prone to shorter attention spans and lower IQ scores than other babies, according to a study in The New England Journal of Medicine.
The condition, called hypothyroidism, affects about 2 out of every 100 women. A fetus's own thyroid gland becomes active during the second trimester and gradually begins producing the hormone on its own, but even then the fetus still depends largely on its mother to supply the hormone, which plays an important role in the developing brain.
Though hypothyroidism is relatively rare, its symptoms -- which include fatigue, constipation, dry skin, a greater sensitivity to cold, and depression -- are difficult to distinguish from pregnancy-related ones. So women with the condition may not be diagnosed, especially since a prenatal screening test isn't routine. (The American Association of Clinical Endocrinologists endorses routine prenatal screenings but other organizations, such as the American College of Obstetricians and Gynecologists, do not.)
Pregnant women should be on the lookout for any of the above symptoms, and check their family history for the disease. If you suspect a problem, ask your doctor to give you a simple blood test to determine your thyroxine level. If your levels are too low, you'll be prescribed a safe hormone pill that you can take daily. Each newborn is screened for hypothyroidism when he’s born.