Thyroid hormone is critical to normal development of the baby’s brain and nervous system. During the first trimester, the fetus depends on the mother’s supply of thyroid hormone, which comes through the placenta. At around 12 weeks, the baby’s thyroid begins to function on its own.
http://www.endocrine.niddk.nih.gov/pubs/pregnancy/
In this article, it says how the baby is strictly dependent on the Mother's thyroid hormone in the first 10-12 weeks.
http://www.thyroid.org/faq-thyroid-disease-and-pregnancy/
Pregnancy and Thyroid Disease Page - National Endocrine and Metabolic Diseases Information Service
www.endocrine.niddk.nih.gov
A service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Institutes of Health (NIH)
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