# Human Reproduction — Part B (NEET Biology Class 12): Menstrual Cycle, Pregnancy & Parturition Source: https://tracknation.in/story/human-reproduction-part-b-neet-biology-class-12-menstrual-cycle-pregnancy-parturition (TrackNation — free UPSC notes with PYQs) > NEET Class 12 Human Reproduction, Part B, mined from NCERT: the menstrual cycle's four phases (with LH surge and corpus luteum), fertilisation (polyspermy block, sex determination), cleavage and implantation (morula, blastocyst, trophoblast), pregnancy and embryonic development (placenta, pregnancy-exclusive hormones, germ layers, trimester milestones), and parturition and lactation (oxytocin positive feedback, colostrum). With 12 MCQs, 6 NEET PYQs and an Active Recall prompt. ## The Menstrual Cycle — Four Phases 🎯 **NEET priority: High-yield.** *Part B of Human Reproduction — the menstrual cycle, fertilisation, pregnancy and parturition. Menstrual cycle phase/hormone questions appear in almost every NEET paper.* The **menstrual cycle** is the reproductive cycle of female primates (monkeys, apes, humans). The first menstruation is **menarche**; cycles repeat roughly every **28–29 days** until **menopause** (~50 years). ### The four phases — quick reference | **Phase** | **Duration (of 28-day cycle)** | **Key event** | **Dominant hormone activity** | | Menstrual | Days 1–5 (3–5 days) | Endometrium sheds — occurs only if NOT fertilised | LH, FSH, estrogen, progesterone all low | | Follicular | Days 6–13 | Follicle grows to Graafian; endometrium regenerates | LH, FSH rise gradually; estrogen rises | | Ovulatory | ~Day 14 | LH surge ruptures Graafian follicle — ovulation | LH and FSH both peak (LH surge) | | Luteal | Days 15–28 | Corpus luteum forms; maintains endometrium | Progesterone high (from corpus luteum) | Now the detail behind each phase, in order: - **Menstrual phase** (**3–5 days**): the endometrial lining breaks down and is shed — occurs **only if the released ovum was NOT fertilised**. - **Follicular phase**: primary follicles grow into a Graafian follicle; endometrium regenerates. **LH and FSH** rise gradually, stimulating follicle development and **estrogen** secretion. - **Ovulatory phase**: LH and FSH both **peak around day 14** — the sharp **LH surge** triggers rupture of the Graafian follicle and **ovulation**. - **Luteal phase**: the ruptured follicle transforms into the **corpus luteum**, which secretes large amounts of **progesterone** — essential to maintain the endometrium for implantation. Without fertilisation, the corpus luteum degenerates → endometrium disintegrates → next menstruation. **During pregnancy, the entire cycle stops** — no menstruation. ## Fertilisation, Cleavage & Implantation ### Fertilisation - Fertilisation needs sperm and ovum to reach the **ampulla****simultaneously** — this is why not every coitus leads to pregnancy. On contact with the **zona pellucida**, the sperm's **acrosome** enzymes help it enter; this contact also triggers membrane changes that **block entry of any other sperm** — ensuring only ONE sperm fertilises the ovum. - Sperm entry triggers the secondary oocyte to **complete its second meiotic division** — again unequal, giving a **second polar body** and a haploid **ovum (ootid)**. The haploid sperm and ovum nuclei fuse into a **diploid zygote**. - **Sex determination:** every ovum carries an **X** chromosome; sperm carry **either X or Y** (50:50). XX → female, XY → male. **The father's sperm determines the baby's sex** — not the mother. ### Cleavage & implantation - As the zygote moves through the **isthmus** toward the uterus, mitotic **cleavage** produces 2 → 4 → 8 → 16 **blastomeres**. The 8–16 cell stage is a **morula**, which develops into a **blastocyst** — an outer **trophoblast** layer and an inner **inner cell mass** (becomes the embryo). - The trophoblast attaches to the endometrium; uterine cells grow rapidly over the blastocyst, embedding it — this is **implantation**, and it marks the start of **pregnancy**. ## Pregnancy & Embryonic Development ### Pregnancy & embryonic development - After implantation, **chorionic villi** from the trophoblast interdigitate with uterine tissue to form the **placenta** — connected to the embryo by the **umbilical cord**. Placenta functions: exchange O₂/CO₂ and nutrients/waste, AND acts as an **endocrine gland**. - Placental/pregnancy hormones: **hCG (human chorionic gonadotropin), hPL (human placental lactogen)** and (from the ovary, late pregnancy) **relaxin** — these three are produced **ONLY during pregnancy**. Levels of estrogens, progestogens, cortisol, prolactin and thyroxine also rise sharply to support fetal growth. - The inner cell mass differentiates into **3 germ layers**: **ectoderm, mesoderm, endoderm** — origin of all adult tissues; it also contains **stem cells**. - **Milestones:** month 1 — heart forms; month 2 (end) — limbs and digits appear; **12 weeks (end of 1st trimester)** — most major organ systems formed, limbs and external genitalia well developed; month 5 — first fetal movements, head hair appears; **24 weeks (end of 2nd trimester)** — body covered in fine hair, eyelids separate; **9 months** — fully developed, ready for delivery. ## Parturition, Lactation & Why This Matters for NEET ### Parturition & lactation - The average **gestation period is 9 months**. Signals from the fully developed foetus and placenta trigger mild uterine contractions — the **foetal ejection reflex** — which releases **oxytocin** from the maternal pituitary. Oxytocin causes stronger contractions, which stimulate MORE oxytocin — a self-reinforcing **positive feedback loop** that ends in **parturition** (childbirth), followed by expulsion of the placenta. - **Lactation** begins near the end of pregnancy. The first milk, **colostrum**, is rich in **antibodies** essential for the newborn's early immunity. ### Embryonic development milestones — the timeline NEET asks | **Stage / time** | **What has developed** | | 1 month | The embryo's HEART is formed — the first sign of a growing foetus, detectable as a heart sound through a stethoscope | | End of 2 months | The foetus develops LIMBS and DIGITS | | End of 12 weeks (1st trimester) | MOST MAJOR ORGAN SYSTEMS are formed; limbs and external genital organs are well developed | | 5 months | First MOVEMENTS of the foetus, and appearance of HAIR on the head | | End of 24 weeks (2nd trimester) | Body covered with FINE HAIR; eyelids SEPARATE; eyelashes formed | | 9 months | Foetus is fully developed and READY FOR DELIVERY | | **Stage of cleavage** | **Description** | | Zygote | Single diploid cell formed by fusion of sperm and ovum nuclei, in the AMPULLA | | Blastomeres (2, 4, 8, 16) | Mitotic cleavage products, formed as the zygote moves through the ISTHMUS toward the uterus | | Morula | The embryo at the 8–16 blastomere stage — a solid ball | | Blastocyst | Morula transforms as it enters the uterus: outer TROPHOBLAST layer + INNER CELL MASS attached to it | | Implantation | Trophoblast attaches to the endometrium; uterine cells divide and cover the blastocyst, embedding it — this MARKS PREGNANCY | ### Why this matters for NEET - High-value one-liners: LH surge ⇒ ovulation around day 14; corpus luteum ⇒ progesterone ⇒ maintains endometrium; menstruation = endometrium shedding when NOT fertilised; hCG/hPL/relaxin are pregnancy-EXCLUSIVE hormones; oxytocin-contraction loop is **positive feedback**, one of NEET's favourite physiology examples of this concept. - **Trap:** the corpus luteum forms from the ruptured Graafian follicle in the **luteal** phase, not the follicular phase. The zona-pellucida block against extra sperm happens at **first** sperm contact — polyspermy prevention, not a post-fertilisation event. A morula (8–16 cells, solid) is NOT the same as a blastocyst (has a trophoblast + inner cell mass + cavity) — don't use them interchangeably. ## Test Yourself: MCQs, PYQs & Active Recall Answer these, then close the article and do an Active Recall. Reveal each answer only after you commit to one. ## Practice Questions **Q1.** The menstrual phase, during which the endometrial lining is shed, lasts about: - (a) 3–5 days - (b) 10 days - (c) 1 day - (d) 14 days **Answer: (a)** — The menstrual phase, marked by menstrual flow, typically lasts 3–5 days. **Q2.** The LH surge that triggers ovulation occurs around day: - (a) 7 - (b) 14 - (c) 28 - (d) 21 **Answer: (b)** — Both LH and FSH peak around day 14 of a typical 28-day cycle; the LH surge ruptures the Graafian follicle. **Q3.** The corpus luteum is formed during the: - (a) Menstrual phase - (b) Follicular phase - (c) Luteal phase - (d) Ovulatory phase **Answer: (c)** — In the luteal phase, the ruptured Graafian follicle transforms into the corpus luteum, which secretes progesterone. **Q4.** Menstruation does NOT occur when: - (a) Fertilisation and pregnancy occur - (b) LH levels are low - (c) FSH levels are low - (d) The ovum is not fertilised **Answer: (a)** — During pregnancy, all events of the menstrual cycle stop, so menstruation does not occur. **Q5.** The mechanism that prevents more than one sperm from fertilising an ovum is triggered at the: - (a) Zona pellucida, on first sperm contact - (b) Second meiotic division only - (c) Uterus, after implantation - (d) Fallopian tube isthmus **Answer: (a)** — Contact of the first sperm with the zona pellucida triggers membrane changes that block entry of additional sperm. **Q6.** The second meiotic division of the oocyte, which is unequal, is completed: - (a) During menstruation - (b) In the fetal ovary - (c) Only after being triggered by sperm entry - (d) Before ovulation **Answer: (c)** — Sperm entry triggers completion of the secondary oocyte's second meiotic division, giving a second polar body and the ovum. **Q7.** The sex of a human baby is determined by: - (a) Hormonal levels during pregnancy - (b) The mother, since she provides the X chromosome - (c) The father, depending on whether the sperm carries X or Y - (d) Both parents equally at conception **Answer: (c)** — All ova carry X; sperm carry X or Y — so the father's sperm determines whether the zygote is XX (female) or XY (male). **Q8.** The solid ball of 8 to 16 blastomeres formed by cleavage is called the: - (a) Zygote - (b) Trophoblast - (c) Blastocyst - (d) Morula **Answer: (d)** — Continued cleavage of the zygote produces the morula (8–16 blastomeres), which later becomes the blastocyst. **Q9.** In the blastocyst, the layer that attaches to the endometrium and later forms the placenta is the: - (a) Inner cell mass - (b) Corpus luteum - (c) Trophoblast - (d) Zona pellucida **Answer: (c)** — The outer trophoblast layer attaches to the endometrium and gives rise to the placenta; the inner cell mass becomes the embryo. **Q10.** Which of these hormones is produced ONLY during pregnancy? - (a) Progesterone - (b) Human chorionic gonadotropin (hCG) - (c) FSH - (d) Estrogen **Answer: (b)** — hCG, along with hPL and relaxin, are produced exclusively during pregnancy; estrogen/progesterone/FSH are also present outside pregnancy. **Q11.** By the end of the first trimester (12 weeks) of human pregnancy: - (a) The baby is ready for delivery - (b) Most major organ systems are formed - (c) The heart has just started forming - (d) Implantation has not yet occurred **Answer: (b)** — By 12 weeks (end of first trimester), most major organ systems, including limbs and external genitalia, are well developed. **Q12.** The self-reinforcing hormonal loop between uterine contractions and oxytocin release during childbirth is an example of: - (a) Feedforward inhibition only - (b) Positive feedback - (c) Negative feedback - (d) No feedback **Answer: (b)** — Stronger contractions stimulate more oxytocin release, which causes even stronger contractions — a positive feedback loop. **Q13.** The sharp mid-cycle surge marked A, which triggers the rupture of the Graafian follicle and release of the ovum, is of - (a) LH - (b) progesterone - (c) oestrogen - (d) FSH **Answer: (a)** — The LH surge (around day 14) induces rupture of the Graafian follicle, causing ovulation. It follows the rising oestrogen, which exerts positive feedback on the pituitary. **Q14.** The hormone marked D, secreted by the corpus luteum and essential for maintaining the endometrium for implantation, is - (a) progesterone - (b) oestrogen - (c) LH - (d) FSH **Answer: (a)** — Progesterone rises after ovulation, peaks in the luteal phase (about day 21) and falls if fertilisation does not occur, causing menstruation. **Q15.** The hormone marked C, secreted by the growing follicles and peaking just before ovulation, is - (a) progesterone - (b) inhibin - (c) oestrogen - (d) relaxin **Answer: (c)** — Oestrogen from the growing follicle peaks around day 12-13; the high level triggers the LH surge (positive feedback). It has a second smaller peak in the luteal phase, from the corpus luteum. ### NEET Previous Year Questions (PYQs) *Real NEET previous-year questions on this chapter, with explanations in our own words.* **Q16.** The corpus luteum secretes large amounts of which hormone essential for maintaining the endometrium? (NEET PYQ) - (a) Estrogen - (b) FSH - (c) Oxytocin - (d) Progesterone **Answer: (d)** — The corpus luteum secretes progesterone, which maintains the endometrium needed for implantation. **Q17.** Which of the following is NOT a hormone produced exclusively during pregnancy? (NEET PYQ) - (a) Relaxin - (b) hPL - (c) hCG - (d) Progesterone **Answer: (d)** — Progesterone is present outside pregnancy too (e.g. luteal phase); hCG, hPL and relaxin are pregnancy-exclusive. **Q18.** The average duration of human gestation (pregnancy) is: (NEET PYQ) - (a) 9 months - (b) 6 months - (c) 3 months - (d) 12 months **Answer: (a)** — Human gestation lasts about 9 months. **Q19.** The structure that provides the vascular connection between foetus and uterus is the: (NEET PYQ) - (a) Chorionic villi alone - (b) Umbilical cord - (c) Zona pellucida - (d) Amniotic sac **Answer: (b)** — The umbilical cord, containing the fetal blood vessels, connects the foetus to the placenta. **Q20.** Colostrum, secreted during the initial days of lactation, is especially rich in: (NEET PYQ) - (a) Minerals only - (b) Antibodies - (c) Fats only - (d) Vitamins only **Answer: (b)** — Colostrum contains antibodies essential for building the newborn's early immunity. **Q21.** Ovulation occurs during which phase of the menstrual cycle? (NEET PYQ) - (a) Menstrual phase - (b) Luteal phase - (c) Follicular phase - (d) Ovulatory phase (around day 14) **Answer: (d)** — Ovulation is triggered by the LH surge at the ovulatory phase, roughly day 14 of a 28-day cycle. ### Active Recall Prompt Write everything you can recall about **Human Reproduction (Part B)**, naming each part first: the **menstrual cycle** (menarche, the four phases in order, LH surge, corpus luteum, menopause); **fertilisation** (zona pellucida block, completion of meiosis II, sex determination); **cleavage and implantation** (morula, blastocyst, trophoblast, inner cell mass); **pregnancy** (placenta, pregnancy-exclusive hormones, germ layers, trimester milestones); and **parturition and lactation** (foetal ejection reflex, oxytocin positive feedback, colostrum). Begin each fact with its topic and end it with a full stop.