NCERT grounding
NCERT Class 12 Biology, Chapter 2 — Human Reproduction — devotes §2.6 to pregnancy and embryonic development, picking up the narrative immediately after implantation. The chapter states that after implantation finger-like chorionic villi appear on the trophoblast, interdigitate with maternal blood-filled uterine tissue to form the placenta, and that the inner cell mass differentiates into ectoderm, mesoderm and endoderm — the three layers that will give rise to every adult organ system. Human pregnancy lasts about 9 months, with explicit milestones at one month (heart), the second month (limbs and digits), the end of the first trimester (most major organs), the fifth month (movements and hair) and the end of the second trimester (eyelid separation, body hair).
NIOS Biology Lesson 21 (§21.2) supplements this with the amnion and amniotic fluid — the embryo is enclosed in a sac filled with shock-absorbing fluid — and explicitly states that the placenta is a tissue through which oxygen and food are supplied from maternal to foetal blood, and that carbon dioxide and excretory wastes move in the reverse direction. Together NCERT §2.6 and NIOS §21.2 cover the entire NEET-testable surface of this subtopic; the present article goes deeper on each line without revisiting fertilisation or implantation, which are treated in their own sibling subtopic.
Pregnancy, the placenta and embryonic development
Pregnancy in humans is a viviparous, internal-development arrangement that lasts approximately 38 weeks from fertilisation (or about 40 weeks from the last menstrual period). The implanted blastocyst that opened pregnancy is a hollow sphere with a single-layered trophoblast and an eccentric inner cell mass. Within hours of implantation the trophoblast issues finger-like outgrowths — the chorionic villi — into the eroded maternal endometrium, opening a foeto-maternal exchange surface. Simultaneously the inner cell mass partitions itself into the three germ layers from which the entire foetal body will be built.
Two parallel constructions therefore run side by side: an extra-embryonic programme that builds the placenta, the umbilical cord, the amnion and the chorion to support the embryo, and an embryonic programme that builds the embryo itself from the inner cell mass. Both are powered by the same blastocyst. The NEET syllabus tests the placenta, the germ layers and the developmental milestones; we treat each in turn.
Weeks of gestation
Human pregnancy lasts about 9 months — measured roughly from fertilisation (38 weeks) or from the last menstrual period (40 weeks). It is conventionally split into three trimesters of 12–13 weeks each.
Germ layers from the inner cell mass
Immediately after implantation the inner cell mass differentiates into an outer ectoderm and an inner endoderm; a third layer, the mesoderm, soon appears between them. NCERT calls these the three layers that give rise to all tissues and organs in adults — a statement NEET examiners have rephrased into matching-list and statement-correct questions for years. The inner cell mass also contains pluripotent stem cells with the potency to form every adult tissue; this single sentence in NCERT seeds many of the biotechnology stem-cell questions in later chapters.
Figure 1. The three germ layers and their organ-system fates. Ectoderm builds outer-body and neural tissues; mesoderm builds connective, muscular and urogenital tissues; endoderm builds the lining of the gut tube and its associated glands.
Memory rule: outside-in — Ectoderm covers (skin + nerves), Mesoderm fills the middle (muscle + bone + blood + kidney + gonad), Endoderm lines the gut tube and its outpushings (liver, pancreas, lungs).
Ectoderm
Derivatives: central & peripheral nervous system, epidermis of skin, hair, nails, lens, retina, inner ear, enamel of teeth.
Also: the amnion (with extra-embryonic mesoderm, NEET 2018).
Asked NEET 2018Mesoderm
Derivatives: skeletal/cardiac/smooth muscle, bones, cartilage, dermis, heart, blood, blood vessels, lymphatics.
Also: kidneys, ureters, gonads, reproductive ducts.
NCERT §2.6Endoderm
Derivatives: epithelial lining of the gut, respiratory tract; liver, pancreas, thyroid, parathyroid, thymus.
Also: urinary bladder lining, lining of middle ear.
NCERT §2.6The placenta — structure and functions
The placenta is the structural and functional unit between the developing foetus and the maternal body. Its anatomy is simple in principle: foetal chorionic villi project from the trophoblast into maternal blood sinuses in the eroded endometrium, so the two circulations come within micrometres of each other without ever mixing. Substances cross by diffusion, facilitated transport and active transport across the thin villous membrane. The placenta connects to the foetus through the umbilical cord, which carries two umbilical arteries (deoxygenated foetal blood to placenta) and one umbilical vein (oxygenated blood back to foetus).
Figure 2. The placenta as a foeto-maternal exchange unit. Foetal chorionic villi dip into maternal blood sinuses; the umbilical cord (two arteries, one vein) connects the placenta to the foetus floating in amniotic fluid.
Functionally the placenta wears four hats. First it is a respiratory organ, transferring oxygen from maternal to foetal blood. Second it is a nutritive organ, supplying glucose, amino acids, lipids, vitamins and water across the villous membrane. Third it is an excretory organ, picking up carbon dioxide, urea and other foetal wastes for the maternal kidneys to deal with. Fourth — and this is where NEET hits hardest — it is an endocrine organ, secreting hCG, hPL, oestrogens and progestogens. The placenta also passes maternal antibodies (IgG) to the foetus and so confers passive immunity for the first few months of postnatal life, although NCERT files most antibody detail under colostrum and parturition.
Placenta vs Umbilical cord — what each does
Placenta
Exchange organ
disc-shaped, embedded in uterine wall
- Chorionic villi bathed in maternal blood sinuses
- Two-way diffusion of gases, nutrients and wastes
- Secretes hCG, hPL, oestrogens, progestogens
- Filters most pathogens (HIV can still cross — NIOS)
- Expelled after delivery as the "afterbirth"
Umbilical cord
Transport pipe
connects placenta to foetus
- Two umbilical arteries: foetus → placenta (deoxygenated)
- One umbilical vein: placenta → foetus (oxygenated)
- Encased in jelly-like Wharton's jelly (mesoderm)
- Tied and cut immediately after birth
- Stump heals as the navel (umbilicus)
Hormones of pregnancy
NEET 2018 Q.136 asked directly: hormones secreted by the placenta to maintain pregnancy are hCG, hPL, progestogens and estrogens — the answer NCERT gives almost word for word. NCERT then adds that the ovary contributes relaxin in the later phase of pregnancy and notes the maternal-blood rise in cortisol, prolactin and thyroxine. The hormones to memorise as pregnancy-only are hCG, hPL and relaxin: NCERT states explicitly that these three are produced in women only during pregnancy.
Month-by-month foetal milestones
NEET examiners love calendar questions and have asked them year after year. The NCERT timeline is short enough to memorise verbatim, and the 2025 paper rewarded students who had done so. The key anchors are: heart at 4 weeks (1 month), limbs and digits by 8 weeks (2 months), most major organ systems by 12 weeks (end of first trimester), foetal movement and hair by ~20 weeks (5th month), eyelid separation and body hair by 24 weeks (end of second trimester), and full-term delivery at ~40 weeks (9 months). A useful supplementary fact: by ~28 weeks the foetal brain has reached about 90% of its eventual weight, which is why the third trimester is so vulnerable to neurological insult.
Foetal milestones — first to ninth month
-
Wk 4
Heart formed
After one month of pregnancy the embryo's heart is formed; its sound is audible on a stethoscope.
~1 month -
Wk 8
Limbs and digits
By the end of the second month the foetus develops limbs and digits — external arms, legs, fingers, toes.
~2 months -
Wk 12
Major organs laid down
By the end of 12 weeks (1st trimester) most major organ systems are formed; external genitalia are well-developed.
end of T1 -
Wk 20
Movement + hair
First foetal movements and appearance of hair on the head — typically observed during the fifth month.
~5 months -
Wk 24
Eyelids & body hair
End of second trimester — body covered with fine hair (lanugo), eyelids separate, eyelashes form.
end of T2 -
Wk 40
Full-term delivery
By the end of nine months the foetus is fully developed and ready for parturition.
term
NEET 2025 Q.157 expanded this list and rewarded the student who could simultaneously affirm that (i) major organ systems are formed by the end of 12 weeks (not 8 weeks), (ii) the heart is formed after one month of gestation, (iii) limbs and digits develop by the end of the second month, and (iv) hair on the head appears in the fifth month. The "8 weeks" version of the major-organ statement was the planted trap — it sounds reasonable but is wrong. Read NCERT's "by the end of 12 weeks" sentence carefully; the trimester boundary is the anchor.
Worked examples
Q. Hormones secreted by the placenta to maintain pregnancy are: (1) hCG, hPL, progestogens, prolactin (2) hCG, hPL, estrogens, relaxin, oxytocin (3) hCG, hPL, progestogens, estrogens (4) hCG, progestogens, estrogens, glucocorticoids.
Answer: (3). NCERT §2.6 states the placenta produces several hormones — hCG, hPL, estrogens, progestogens — and that relaxin is secreted by the ovary (not the placenta) in the later phase of pregnancy. Option (1) is wrong because prolactin is pituitary, not placental; option (2) wrongly assigns relaxin and oxytocin to the placenta; option (4) wrongly assigns glucocorticoids to placental secretion.
Q. Match the germ layer with one of its derivatives: (a) Ectoderm (b) Mesoderm (c) Endoderm with (i) Liver (ii) Kidney (iii) Epidermis.
Answer: (a)–(iii), (b)–(ii), (c)–(i). Ectoderm → epidermis (outermost covering); mesoderm → kidney (middle layer builds urogenital system); endoderm → liver (an outgrowth of the gut tube, which is lined by endoderm). The rule is "outside-in": skin/nerves, then muscle/skeleton/kidney, then gut/liver/lung.
Q. Which of the following is correct? (A) Major organ systems form by 8 weeks (B) Major organ systems form by 12 weeks (C) Heart forms after one month (D) Limbs and digits develop by the end of the second month (E) Hair appears in the fifth month.
Answer: B, C, D and E are correct; A is wrong. NCERT explicitly says "by the end of 12 weeks (first trimester) most of the major organ systems are formed." The trap of substituting "8 weeks" for "12 weeks" is the single most asked trick in this subtopic (see NEET 2025).
Q. The amnion of mammalian embryo is derived from: (1) ectoderm and mesoderm (2) endoderm and mesoderm (3) mesoderm and trophoblast (4) ectoderm and endoderm.
Answer: (1). The amnion is formed by ectoderm plus extra-embryonic mesoderm — an anatomy-of-extra-embryonic-membranes fact asked by NEET 2018 Q.139. The amniotic fluid it encloses acts as a hydraulic shock-absorber and allows free foetal movement.