🤰 Reproductive System Lesson

hCG → Progesterone → Estrogen → Oxytocin → Prolactin

Five hormones, each dominant at a different stage — from confirming pregnancy to delivering and feeding the baby.

hCG

Early

Prog.

Maintain

Estr.

Growth

Oxyt.

Labor

Prol.

Milk

📖 Full Breakdown

Five hormones in sequence, each with a distinct role across pregnancy and delivery

One hormone in this sequence works through positive feedback — the opposite mechanism from how most hormones in the body are regulated.

hCG

The first hormone of pregnancy

Maintains the corpus luteum, peaks around week 10, then declines as the placenta takes over hormone production. Morning sickness severity often correlates with hCG levels.

Progesterone

Maintains the pregnancy

Prevents uterine contractions, maintains the endometrium, and suppresses immune rejection of the fetus — a genuinely remarkable function, since the fetus is genetically distinct tissue that the mother's immune system might otherwise attack.

Estrogen

Drives growth

Promotes uterine growth and breast development, rising steadily throughout pregnancy.

Oxytocin

Labor and lactation, via positive feedback

Drives labor contractions and milk letdown. Unlike most hormones, oxytocin works through POSITIVE feedback during labor — more contractions trigger more oxytocin release, which triggers even more contractions, escalating rather than self-correcting.

Prolactin

Milk production

Actively inhibited by estrogen during pregnancy itself, only being released in significant amounts after delivery once estrogen levels drop.

🩺 Clinical / Exam Application

During labor, a woman's contractions become progressively stronger and more frequent as delivery approaches, rather than staying constant or gradually fading. This escalating pattern happens because oxytocin, unlike nearly every other hormone in the body, operates through POSITIVE feedback during labor: each contraction triggers more oxytocin release, and that oxytocin triggers even stronger contractions — a self-reinforcing loop that intentionally builds in intensity until delivery occurs, rather than being dampened by the negative feedback mechanisms that regulate most other hormonal systems.

⚠️ Exam Alert

Oxytocin's positive feedback mechanism during labor is one of the most frequently tested exceptions to the general rule that most hormones use negative feedback — exam questions often specifically test whether you can identify this hormone as a rare positive-feedback example.

🚧 Common Trap

Don't assume prolactin is active throughout pregnancy just because it produces milk. Prolactin is actively SUPPRESSED by estrogen during pregnancy itself — it only rises significantly after delivery, once estrogen levels fall, which is exactly why milk production typically doesn't begin in earnest until after birth.

✅ Quick Check

Why do labor contractions typically get stronger and more frequent as delivery approaches, rather than staying at a constant intensity?

📝 Exam Prep

Common Exam Questions

❓ What is the sequence of hormones involved in pregnancy and lactation?

✅ hCG (maintains corpus luteum, peaks week 10) → Progesterone (maintains pregnancy, suppresses immune rejection) → Estrogen (uterine and breast growth) → Oxytocin (labor contractions, milk letdown) → Prolactin (milk production, released after delivery).

❓ Why is oxytocin's role in labor described as positive feedback?

✅ Unlike most hormones which use negative feedback to self-correct, oxytocin release during labor is amplified by each contraction — more contractions trigger more oxytocin, which triggers even stronger contractions, escalating rather than stabilizing.