The trap: NEET repeatedly tests whether you confuse the hormonal trigger of parturition with the hormone that maintains pregnancy. Progesterone maintains pregnancy — it does NOT trigger labour. Oxytocin triggers uterine contractions via positive feedback. Getting this backward costs you marks under negative marking.
Fertilisation and early development. Fertilisation occurs in the ampulla (ampullary-isthmic junction) of the fallopian tube (NCERT Class 12 Biology Chapter 2, page 35). The zygote undergoes cleavage divisions during its transit through the oviduct, forming a blastocyst. Implantation occurs approximately 6–7 days after fertilisation, when the blastocyst embeds in the uterine endometrium. The trophoblast layer establishes contact with the endometrium for nutrient exchange.
Pregnancy. After implantation, the chorion develops finger-like villi that interdigitate with uterine tissue to form the placenta. The placenta secretes human chorionic gonadotropin (hCG) — not the anterior pituitary. hCG maintains the corpus luteum during the first trimester so that progesterone production continues. Later, the placenta itself produces progesterone, oestrogen, and human placental lactogen (hPL).
Parturition. At term (~9 months), signals from the foetus (foetal cortisol) initiate a cascade. Oxytocin from the posterior pituitary stimulates uterine smooth muscle contractions. These contractions stimulate more oxytocin release — a positive feedback loop (neuroendocrine reflex). Prostaglandins amplify contractions. Progesterone levels fall before parturition; it does not drive contractions.
Lactation. Prolactin (anterior pituitary) stimulates milk production. Oxytocin triggers milk ejection (let-down reflex). Colostrum, the first secretion, contains antibodies (IgA) providing passive immunity to the newborn.
Watch-out: hCG source = placenta (not pituitary). Parturition trigger = oxytocin (not progesterone). Implantation timing = day 6–7 post-fertilisation.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Where does fertilisation normally occur in the human female reproductive tract?
Answer: A. Fertilisation occurs at the ampullary-isthmic junction of the oviduct (NCERT Class 12 Biology Chapter 2, page 30).
Why B is wrong: B is wrong because the uterine cavity is the site of implantation, not fertilisation — confusing location of two distinct events.
Why C is wrong: C is wrong because the cervical canal is merely the passage for sperm entry from vagina to uterus; fertilisation occurs higher in the tract.
Why D is wrong: D is wrong because fimbriae collect the ovum from the ovary surface but fertilisation occurs within the ampulla, not at the fimbriae.
Implantation of the human blastocyst in the uterine endometrium occurs approximately:
Answer: B. The blastocyst implants in the endometrium approximately 6–7 days after fertilisation (NCERT Class 12 Biology Chapter 2, page 38). The intervening days are spent in cleavage divisions during oviductal transit.
Why A is wrong: A is wrong because 24 hours post-fertilisation the zygote has undergone only its first cleavage; it is still in the oviduct, not the uterus (trap: implantation day).
Why C is wrong: C is wrong because at 3 days the embryo is at the morula stage, still transiting through the oviduct and has not yet reached the uterus.
Why D is wrong: D is wrong because day 14 corresponds to ovulation timing in the menstrual cycle, not implantation — confusing menstrual timeline with post-fertilisation timeline.
Human chorionic gonadotropin (hCG) is secreted by:
Answer: A. hCG is secreted by the chorionic villi (trophoblast/placenta), not the pituitary. Its function is to maintain the corpus luteum during early pregnancy (NCERT Class 12 Biology Chapter 2, page 37).
Why B is wrong: B is wrong because the corpus luteum is the TARGET maintained by hCG, not the source of hCG secretion.
Why C is wrong: C is wrong because the anterior pituitary secretes LH and FSH, not hCG — a common confusion since both LH and hCG act on the corpus luteum (mistake: mistake: placenta hormone).
Why D is wrong: D is wrong because the hypothalamus releases GnRH, not hCG; it regulates the pituitary, not the placenta.
Which hormone is primarily responsible for triggering uterine contractions during parturition?
Answer: C. Oxytocin from the posterior pituitary triggers uterine smooth muscle contractions via a positive feedback mechanism. Foetal cortisol initiates the cascade, and prostaglandins amplify contractions (NCERT Class 12 Biology Chapter 2).
Why A is wrong: A is wrong because progesterone MAINTAINS pregnancy by suppressing uterine contractions; it falls before parturition rather than triggering it (mistake: mistake: parturition hormone).
Why B is wrong: B is wrong because oestrogen increases uterine sensitivity to oxytocin but is not the direct trigger of labour contractions.
Why D is wrong: D is wrong because prolactin stimulates milk production (lactogenesis), not uterine contractions.
During parturition, oxytocin release increases progressively because of:
Answer: D. Parturition involves a neuroendocrine positive feedback loop: uterine contractions stretch the cervix → signals to hypothalamus → more oxytocin released → stronger contractions → cycle continues until delivery (NCERT Class 12 Biology Chapter 2).
Why A is wrong: A is wrong because negative feedback would REDUCE oxytocin secretion; parturition requires escalating contractions, which is characteristic of positive feedback.
Why B is wrong: B is wrong because prolactin governs lactation and has no role in stimulating oxytocin release during parturition.
Why C is wrong: C is wrong because while progesterone does fall before labour, the progressive increase in oxytocin during labour is driven by the positive feedback reflex, not by progesterone inhibition at the pituitary.
The function of hCG during early pregnancy is to:
Answer: B. hCG maintains the corpus luteum during the first trimester, ensuring continued progesterone production that supports the endometrium and prevents menstruation (NCERT Class 12 Biology Chapter 2).
Why A is wrong: A is wrong because milk production is stimulated by prolactin, not hCG; hCG's role is strictly maintaining the corpus luteum.
Why C is wrong: C is wrong because hCG mimics LH's corpus-luteum-maintaining action; it does not trigger new ovulation (in fact, continued progesterone suppresses the next cycle's ovulation).
Why D is wrong: D is wrong because endometrial shedding occurs when progesterone FALLS; hCG's role is precisely to prevent this by maintaining progesterone levels.
A student claims that progesterone triggers labour. Which of the following corrections is accurate?
Answer: C. Progesterone maintains pregnancy by keeping the myometrium quiescent. Before parturition, progesterone withdrawal occurs and oxytocin (with prostaglandins) drives contractions. The student's error is a common confusion between the maintenance hormone and the triggering hormone (NCERT Class 12 Biology Chapter 2).
Why A is wrong: A is wrong because progesterone SUPPRESSES uterine contractions during pregnancy; it cannot simultaneously trigger them via positive feedback (mistake: mistake: parturition hormone).
Why B is wrong: B is wrong because progesterone levels FALL (not rise) before parturition; a rise would further suppress contractions and delay labour.
Why D is wrong: D is wrong because progesterone and oxytocin are functionally antagonistic regarding uterine contractions — progesterone inhibits, oxytocin stimulates.
Which of the following statements about lactation is correct?
Answer: D. Colostrum, the initial milk secreted after delivery, is rich in IgA antibodies that confer passive immunity to the newborn (NCERT Class 12 Biology Chapter 2).
Why A is wrong: A is wrong because prolactin stimulates milk PRODUCTION (synthesis), not ejection; milk ejection (let-down) is triggered by oxytocin.
Why B is wrong: B is wrong because oxytocin triggers milk EJECTION by contracting myoepithelial cells around alveoli; milk production is governed by prolactin.
Why C is wrong: C is wrong because functional lactation begins AFTER delivery; during pregnancy, high progesterone and oestrogen actually inhibit full lactation despite prolactin's presence.
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Given
• Early pregnancy (week 6)• Corpus luteum removed surgically• Placenta not yet fully functional for steroid production
Required
Predict the consequence of corpus luteum removal at this stage.
Concept
During weeks 1–8 of pregnancy, the corpus luteum is the primary source of progesterone. hCG (from the placenta) maintains the corpus luteum. Progesterone maintains the endometrium and suppresses uterine contractions. If progesterone supply ceases before the placenta assumes production (~week 8–12), the endometrium cannot be maintained.
Key relationships
• hCG → maintains corpus luteum → progesterone secretion• Progesterone → maintains endometrium → sustains pregnancy• Remove corpus luteum at week 6 → progesterone drops → endometrium degenerates
Reasoning chain
Corpus luteum removed → no progesterone source (placenta not ready) → endometrial breakdown → spontaneous abortion (miscarriage).
Conclusion
The most likely consequence is spontaneous abortion due to progesterone withdrawal, since the placenta has not yet taken over progesterone production at week 6.
Final answer
Removal of corpus luteum at week 6 leads to spontaneous abortion because the placenta cannot yet sustain adequate progesterone levels to maintain the endometrium.
Common trap
Students may say "hCG levels drop" — but hCG is from the placenta, not the corpus luteum. Removing the corpus luteum does not directly reduce hCG; it removes the TARGET of hCG. The consequence is progesterone loss, not hCG loss. Also, confusing this with late pregnancy (when the placenta is self-sufficient) leads to the wrong answer — after week 12, corpus luteum removal has minimal effect.
Similar NEET-style question
"If hCG secretion were blocked during week 4 of pregnancy, what would be the expected outcome?" (Answer: corpus luteum degenerates → progesterone falls → miscarriage — same reasoning chain but triggered one step earlier.)
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Fertilization → zygote → cleavage → blastocyst → implantation (~6-7 days). hCG maintains corpus luteum first 3 months. Placenta develops; oxytocin → parturition. Prolactin → milk; oxytocin → milk ejection. Colostrum rich in IgA antibodies.
-- NCERT Class 12 Biology, Ch. 2, p. 37These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Negative Marking
Fertilisation in ampulla; cleavage during journey; implantation occurs ~6-7 days after fertilisation in uterine endometrium.
Question on day of implantation, location of fertilisation.
Fertilisation = ampulla. Implantation = day 6-7 post-fertilisation in uterus.
Root cause: term confusion
Progesterone MAINTAINS pregnancy; falls before parturition. Oxytocin (positive feedback) drives uterine contractions. Foetal cortisol triggers cascade.
Root cause: term confusion
hCG = human Chorionic Gonadotropin — placenta. LH/FSH = anterior pituitary. hCG maintains corpus luteum during early pregnancy.
More in Reproduction: 7 exam traps and mistakes · 1 question pattern from its other lessons.
7 questions from NEET 2021, 2023, 2025, 2026. Answers verified against NTA official keys.
Which of the following hormone is not secreted by human placenta ?
Colostrum, secreted by mother during initial days of lactation, is abundant in ______.
The covering of ovum at ovulation is ______.
Receptors for sperm binding in mammals are present on :
Which of the following secrets the hormone, relaxin, during the later phase of pregnancy?
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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