Insights:

The placenta needs to make oestrogens and progesterone to maintain pregnancy and prepare mother for postpartum period. The fetus needs to make cortisol for organ maturation.

Fetus cannot make oestrogens and progesterons, but placenta can. Placenta has 3betaHSD that can make progesterone from pregnenolone, which is essential for the fetal adrenal cortex to make cortisol in early gestation. In late gestation, high levels of progesterone and cortisol are part of the stimulus for partitution.

Conversely, fetal adrenal cortex has 17 alpha hydroxylase and 17,20 desmolase which make DHEA from pregnenolone. 16 hydroxylase in fetal liver makes 16-OH-DHEA. Placenta has aromatase which can convert these fetal DHEAs into oestrogens. Oestrogens are vital for maintaining pregnancy and preparing mum for postpartum period (e.g. by proliferating mammary glands).

DHEA levels spike early in gestation and contribute to genital differentiation, but stay relatively low until 10 years of age (adrenarche).

Cortisol levels continue to rise in gestation as fetal adrenal cortex matures, finally contributing to parturition, via a feed-forward loop with placental CRH.

Reference

Moore, K., Persaud, T. and Torchia, M. (n.d.). Before we are born. 9th ed.

Polin, R., Abman, S., Rowitch, D., Benitz, W. and Fox, W. (n.d.). Fetal and neonatal physiology.