The Placenta — Lifeline Between Mother and Fetus

After implantation, the trophoblast develops into the placenta — a unique organ that forms the interface between the mother and the developing fetus. The placenta is essentially temporary — it forms during pregnancy and is delivered after the baby (as the "afterbirth").

What is the placenta?

The placenta is a disc-shaped organ that:

  • Connects the fetus to the uterine wall.
  • Provides nutrition, gas exchange, waste removal, and hormone production.
  • Develops from BOTH the trophoblast (fetal contribution) and endometrium (maternal contribution).

Two parts of the placenta

Part Origin Composition
Fetal part (chorion) Trophoblast Chorionic villi
Maternal part (decidua) Endometrium Modified uterine tissue

Definition (NCERT-canonical)

Placenta = a structural and functional unit between the developing embryo (fetus) and maternal body, formed from the chorionic villi (fetal) embedded in the maternal endometrium.

\Structure of the human placenta

Functions of the Placenta

The placenta performs 5 critical functions for the developing fetus. Memorise all 5:

1. Nutrition

  • Glucose, amino acids, fatty acids, vitamins, minerals pass from mother to fetus.
  • Mostly via simple diffusion + active transport across placental membrane.

2. Gas exchange

  • Oxygen (O₂) diffuses from maternal blood → fetal blood.
  • Carbon dioxide (CO₂) diffuses from fetal blood → maternal blood.
  • The placenta acts as the fetal lungs (since fetal lungs aren't functional in utero).

3. Waste removal

  • Urea, uric acid, and other metabolic waste from the fetus are transferred to maternal blood.
  • The mother's kidneys excrete these wastes through her urine.
  • The placenta acts as the fetal kidneys.

4. Hormone production (endocrine function)

The placenta is itself an endocrine gland, producing several hormones:

  • hCG (human Chorionic Gonadotropin) — first 3 months; maintains corpus luteum.
  • hPL (human Placental Lactogen) — prepares mammary glands for lactation; alters maternal metabolism.
  • Oestrogen (estriol mainly) — supports uterine growth, mammary preparation.
  • Progesterone — maintains endometrium; suppresses uterine contractions.
  • Relaxin (secreted by the ovary, not the placenta — NCERT) — relaxes the pelvic ligaments and softens the cervix (later in pregnancy).

After ~3 months, the placenta takes over hormone production from the corpus luteum.

5. Immunological barrier

  • Prevents the mother's immune system from attacking the fetus (which is genetically half-foreign).
  • Some maternal antibodies (IgG) can cross — providing passive immunity to the newborn.

What does NOT cross

  • Most bacteria and large viruses are blocked.
  • Maternal blood itself doesn't mix with fetal blood (they are separated by placental membrane).
  • Some drugs and toxins can cross — important in pregnancy (alcohol, nicotine, certain medications must be avoided).

[NEET Important] The 5 functions: Nutrition + Gas exchange + Waste removal + Hormone production + Immunological barrier.

Pregnancy Hormones — A Cocktail of Change

Pregnancy is sustained by a cocktail of hormones, mostly produced by the placenta (after the corpus luteum's role declines).

1. hCG (human Chorionic Gonadotropin)

  • Source: Trophoblast → later placenta.
  • Time: Detectable from ~day 7-10; peaks at ~10 weeks; declines later.
  • Function:
  • Maintains corpus luteum for ~3 months (so progesterone continues).
  • Basis of pregnancy tests (detected in urine).
  • Importance: Without hCG, pregnancy can't continue past the first cycle.

2. hPL (human Placental Lactogen, also called hCS)

  • Source: Placenta.
  • Function:
  • Prepares mammary glands for lactation.
  • Alters maternal metabolism to make more glucose available for fetus.
  • Increases insulin resistance in mother (so glucose stays in blood for fetus).

3. Oestrogen (mainly Estriol)

  • Source: First placenta + maternal adrenal glands; later mainly placenta.
  • Function:
  • Promotes uterine growth (uterus expands ~500-1000× during pregnancy).
  • Prepares mammary glands.
  • Promotes maternal blood vessel development.

4. Progesterone

  • Source: First corpus luteum (3 months); later placenta.
  • Function:
  • Maintains endometrium (key for pregnancy).
  • Suppresses uterine contractions (prevents premature labour).
  • Suppresses ovulation (no new cycles during pregnancy).

5. Relaxin (from the ovary, later in pregnancy)

  • Source: the ovary, in the later phase of pregnancy (NCERT).
  • Function:
  • Relaxes pelvic ligaments to allow childbirth.
  • Softens the cervix to prepare for dilation during labour.

Summary table

Hormone Source Function
hCG Trophoblast → placenta Maintains corpus luteum (3 months); pregnancy test
hPL Placenta Mammary preparation; metabolic shifts
Oestrogen Placenta Uterine growth; mammary preparation
Progesterone Corpus luteum → placenta Endometrium maintenance; suppress contractions
Relaxin Ovary (later phase of pregnancy) Relaxes pelvic ligaments; softens cervix

[NEET Important] hCG, hPL and relaxin are produced in women only during pregnancy; estrogens, progestogens, cortisol, prolactin and thyroxine rise several-fold in maternal blood (NCERT). hCG is the most-tested hormone of this group.

Small Memory Capsule

Section 12 in 60 seconds.

Placenta = mother-fetus interface

  • Origin: Trophoblast (fetal) + endometrium (maternal)
  • Two parts: Chorion (fetal) + Decidua (maternal)
  • Connection: Fetus connects via umbilical cord (2 umbilical arteries + 1 umbilical vein)

5 functions

  1. Nutrition — glucose, amino acids, etc. cross to fetus
  2. Gas exchange — O₂ in, CO₂ out (placenta = fetal lungs)
  3. Waste removal — urea, uric acid (placenta = fetal kidneys)
  4. Hormone production — hCG, hPL, oestrogen and progesterone — while relaxin comes from the ovary in late pregnancy
  5. Immunological barrier — protects fetus; maternal antibodies cross (passive immunity)

5 pregnancy hormones (memorise!)

Hormone Source Key function
hCG Trophoblast/placenta Maintains corpus luteum; pregnancy test
hPL Placenta Mammary preparation + metabolic shift
Oestrogen Placenta Uterine growth
Progesterone Corpus luteum/placenta Maintain endometrium; suppress contractions
Relaxin Ovary (late pregnancy) Relax pelvic ligaments; soften cervix

Hormone timing

  • First 3 months: corpus luteum is main hormone source (under hCG support).
  • After 3 months: placenta takes over all hormone production.
  • hCG peaks ~10 weeks, then declines.
  • Estrogen + progesterone rise throughout pregnancy.

Maternal-fetal blood

  • DO NOT mix directly!
  • Separated by placental membrane.
  • Exchange via diffusion + active transport.

One-line takeaway

"Placenta = temporary organ from chorion + decidua; provides nutrition, gas exchange, waste removal, hormone production (hCG, hPL, oestrogen and progesterone — while relaxin comes from the ovary in late pregnancy), and acts as immunological barrier; maternal and fetal blood don't mix but exchange substances across the placental membrane."

Solved Examples

Example 1: Define placenta

What is the placenta? Where does it come from?

Solution:

The placenta is a disc-shaped, temporary organ that forms during pregnancy as the interface between the developing fetus and the maternal body.

Origin:

  • Fetal part (Chorion): From the trophoblast of the blastocyst. Forms chorionic villi.
  • Maternal part (Decidua): From the endometrium of the uterus. Modified during pregnancy.

The placenta develops over the first 3 months of pregnancy and remains active until birth, when it's delivered as the afterbirth.

Answer: Placenta = mother-fetus interface; fetal chorion + maternal decidua.

Example 2: 5 functions

List the five major functions of the placenta.

Solution:

  1. Nutrition — transfer of glucose, amino acids, fatty acids, vitamins, minerals from mother to fetus.

  2. Gas exchange — O₂ from mother → fetus; CO₂ from fetus → mother (placenta = "fetal lungs").

  3. Waste removal — urea, uric acid, etc. from fetus → mother for excretion (placenta = "fetal kidneys").

  4. Hormone production — hCG, hPL, oestrogen and progesterone — while relaxin comes from the ovary in late pregnancy.

  5. Immunological barrier — protects fetus from infection while allowing some maternal antibodies (IgG) to cross for passive immunity.

Answer: Nutrition + Gas exchange + Waste removal + Hormones + Immunological barrier.

Example 3: Hormone identification

What hormone is detected by urinary pregnancy tests? Where is it produced?

Solution:

hCG (human Chorionic Gonadotropin) is detected by urinary pregnancy tests.

  • Production: Trophoblast cells of the blastocyst (and later the placenta).
  • First detectable: ~7-10 days post-fertilisation in blood; ~10-14 days in urine.
  • Function: Maintains the corpus luteum for ~3 months → progesterone continues → endometrium maintained → pregnancy continues.
  • Peak: ~10 weeks of pregnancy.
  • Decline: After 10 weeks, placenta takes over hormone production from corpus luteum, hCG levels decline.

A positive urine pregnancy test detects hCG → confirms the embryo has implanted.

Answer: hCG; produced by trophoblast/placenta; basis of pregnancy tests.

Example 4: Placental hormones

Name 4 hormones produced by the placenta and their functions.

Solution:

  1. hCG — Maintains corpus luteum (~3 months); basis of pregnancy tests.
  2. hPL (human Placental Lactogen) — Prepares mammary glands for lactation; alters maternal metabolism (increases glucose availability for fetus).
  3. Oestrogen — Promotes uterine growth (~500-1000× expansion); mammary preparation.
  4. Progesterone — Maintains endometrium; suppresses uterine contractions (preventing premature labour).
  5. Relaxin (from the ovary, late pregnancy — NCERT) — relaxes pelvic ligaments and softens the cervix.

Answer: hCG, hPL, oestrogen and progesterone — while relaxin comes from the ovary in late pregnancy (5 hormones; can also list 4).

Example 5: Maternal-fetal blood mixing

Do maternal and fetal blood mix in the placenta?

Solution:

NO — maternal and fetal blood DO NOT mix.

They are separated by the placental membrane (also called the placental barrier), which is composed of:

  1. Fetal capillary endothelium
  2. Connective tissue (fetal)
  3. Trophoblast (fetal)
  4. Maternal blood pools (in chorionic spaces)

Exchange occurs across this membrane by:

  • Diffusion (gases, small molecules)
  • Active transport (glucose, amino acids, ions)
  • Endocytosis/exocytosis (some larger molecules)

Why is this important?

If maternal and fetal blood mixed:

  • Blood type compatibility issues (e.g., Rh incompatibility could be catastrophic).
  • Maternal immune cells would attack the fetus.
  • Pathogens could pass directly.

The separation by the placental membrane allows essential exchange while maintaining biological compatibility.

Answer: No, they don't mix. Separated by placental membrane; only specific substances cross via diffusion/active transport.

Example 6: Why does pregnancy stop menstruation?

Why doesn't a pregnant woman menstruate?

Solution:

Pregnancy stops menstruation through a hormone cascade:

  1. Implanted blastocyst's trophoblast produces hCG.
  2. hCG maintains the corpus luteum — so it doesn't degenerate at day 28 as it normally would.
  3. Corpus luteum continues producing progesterone for ~3 months.
  4. Progesterone maintains the endometrium — so it doesn't shed.
  5. No menstruation occurs.

After ~3 months, the placenta takes over progesterone production. Endometrium continues to be maintained throughout pregnancy.

So the first sign of pregnancy for many women is a missed period (~day 32-35 of cycle), because the cycle that should have ended in menstruation continues into pregnancy.

Answer: hCG → maintains corpus luteum → progesterone continues → endometrium not shed → no menstruation throughout pregnancy.