At what stage of pregnancy does the fetal thyroid gland become functional?

The correct answer is that the fetal thyroid gland becomes functional by the end of the first trimester. At around 10-12 weeks of gestation, the fetal thyroid gland begins to develop and is capable of synthesizing thyroid hormones. This is a critical time for fetal development, as thyroid hormones play a significant role in regulating metabolism and are essential for brain development and growth.

While some thyroid hormone activity exists earlier in gestation, its meaningful production and regulation do not become functional until the end of the first trimester. This developmental milestone is crucial because adequate levels of thyroid hormones are necessary for the ongoing development of the fetus, particularly the central nervous system and other vital organs.

The other options involve periods when the thyroid gland has either not yet developed its function or is not the primary source of hormones influencing fetal development. Understanding this timeline is essential for recognizing the importance of thyroid hormones in fetal growth and health.