A gland that grows, works, and is discarded
Most endocrine glands are permanent and small. The placenta is the opposite: it is built from scratch each pregnancy, grows to roughly half a kilogram, secretes enormous quantities of hormones, and is then expelled at birth. For nine months it acts as a full-blown endocrine organ — and also as lung, kidney, and gut for the fetus. Here we focus on its hormonal job.
After the corpus luteum hands over (the luteal-placental shift from guide 1), the placenta becomes the main source of two steroids that define pregnancy: progesterone and the estrogens. It also makes its own peptide hormones, including human placental lactogen, which the next guide covers.
Progesterone: keeping the uterus quiet
The placenta can make progesterone on its own from maternal cholesterol — it does not need the fetus for this step. Its main job is to keep the uterine muscle relaxed and unresponsive so that the pregnancy is not expelled too early. Progesterone is, in a sense, the hormone of staying pregnant: high levels mean a quiet uterus. You will see in guide 4 that labor cannot begin until this progesterone-dominated state is overridden.
Estrogen: a job neither can do alone
Estrogen synthesis is the placenta's most surprising trick. The placenta lacks one enzyme it needs to make androgen precursors, and the fetus lacks a different enzyme needed later in the pathway. Neither can make estrogen alone — but together they can. Mother, placenta, and fetus pass intermediates back and forth like a relay, a cooperative system called the [[fetal-placental-unit|fetal-placental unit]].
FETAL-PLACENTAL UNIT (estrogen relay, simplified)
MOTHER FETUS PLACENTA
cholesterol --> adrenal makes DHEA-S
liver adds OH group
| (16-OH-DHEA-S)
+-----------------> aromatase
|
v
ESTRIOL (E3)
Key point: estriol needs the FETAL adrenal + liver AND the
placental aromatase. Neither side can finish the molecule alone.
-> rising maternal estriol once tracked fetal well-being.The dominant estrogen of pregnancy is estriol (E3), which is barely present otherwise. Because making it requires a healthy fetal adrenal gland and liver plus placental aromatase, a steadily rising maternal estriol was historically read as a sign that the fetus was thriving. Estrogens also stimulate growth of the uterus and the breast, preparing the body for both birth and feeding.