At approximately day 1

Fetal pituitary responds to thyroid-releasing hormone (TRH) from the hypothalamus.

Duration: 15 weeks

Reference [PubMed Link]

Howdeshell KL. 2002. A model of the development of the brain as a construct of the thyroid system. Environ Health Perspect 110 (suppl 3):337-348.

At approximately day 1

Fetal cortex grows in size; cortical cells attain maximal size late in gestation. There is a marked increase in fetal adrenal 3beta-HSD activity, which indicates an increase in serum concentrations of cortisol.

Duration: 13 weeks

Reference [PubMed Link]

Pepe GJ, Albrect ED. 1990. Regulation of the primate fetal adrenal cortex. Endcrine Reviews 11(1):151-176.