In pregnancy, when is the greatest danger to the fetus from maternal hypothyroidism?

• A. First trimester
• B. Second trimester
• C. Third trimester
• D. Postpartum

Answer: (A)

The most vulnerable period is early in pregnancy because this is when brain development and organ formation are occurring, and the fetus relies almost entirely on maternal thyroid hormone. The fetal thyroid doesn’t become fully functional until later, so insufficient maternal thyroid hormone during the first trimester can lead to irreversible neurodevelopmental problems and growth issues. After the first trimester, the fetus begins producing thyroid hormone itself, so the relative impact of maternal hypothyroidism on neurodevelopment is reduced, though severe or untreated hypothyroidism later still poses risks.