Human Reproduction

This is the second chapter of NCERT's Class 12 Biology textbook (pages 26–40 of the 2026-27 reprint) in Unit VI, Reproduction. It has seven parts — the male reproductive system (2.1), the female reproductive system (2.2), gametogenesis (2.3), the menstrual cycle (2.4), fertilisation and implantation (2.5), pregnancy and embryonic development (2.6), and parturition and lactation (2.7) — with Figures 2.1–2.12, a Menstrual Hygiene box, a summary and 21 exercise questions. The language is clinical; this page keeps it that way.

• Neighbouring chapters: the previous chapter, Sexual Reproduction in Flowering Plants, follows the same logic in plants — meiosis to make gametes, fertilisation, an embryo fed by stored food. Compare: pollen grain ↔ sperm, embryo sac ↔ ovum, endosperm ↔ placenta as the embryo's food supply. The next chapter, Reproductive Health, covers contraception, medical termination of pregnancy, sexually transmitted infections and assisted reproductive technologies in detail. On this site, Population and sociology chapters discuss the sex ratio and gender.
• Prelims — science: UPSC has asked where fertilisation happens (2007), what the corpus luteum is and where it is found (1997), who determines a baby's sex (1981, 1984, 2000), how identical twins arise (1981) and when the embryo's heart starts beating (1994).
• GS1 and GS2 — society and health: the biology of sex determination answers the old practice of blaming mothers for daughters, a root of son preference, sex-selective abortion and an adverse child sex ratio; reproductive and menstrual health are health-policy themes. GS3: stem cells, assisted reproduction and its regulation. Laws are summarised in Section 10 (Beyond NCERT).

• Male: testes in the scrotum (2–2.5 °C cooler); about 250 lobules × 1–3 seminiferous tubules; Sertoli cells nourish, Leydig cells make androgens. Ducts: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra. Glands: seminal vesicles, prostate, bulbourethral.
• Female: ovaries (2–4 cm); fallopian tube (10–12 cm: infundibulum with fimbriae, ampulla, isthmus); uterus with perimetrium, myometrium, endometrium; cervix and vagina (birth canal); mammary glands with 15–20 lobes.
• Spermatogenesis (from puberty): spermatogonium (46) → primary spermatocyte (46) → meiosis I → 2 secondary (23) → meiosis II → 4 spermatids → spermiogenesis → sperms → spermiation. GnRH → LH (Leydig → androgens) + FSH (Sertoli).
• Oogenesis starts in the fetal ovary; primary oocytes wait in prophase I; meiosis I ends in the tertiary follicle (secondary oocyte + first polar body); meiosis II ends only at sperm entry. One primary oocyte → one ovum.
• Menstrual cycle about 28/29 days: menses 3–5 days → follicular (proliferative) → LH surge, ovulation about day 14 → luteal (corpus luteum, progesterone). No fertilisation → corpus luteum degenerates → menstruation.
• Fertilisation in the ampulla; the sperm decides the sex (X or Y). Cleavage → morula (8–16) → blastocyst (trophoblast + inner cell mass) → implantation. Placenta: exchange + hormones (hCG, hPL); birth by an oxytocin positive-feedback loop; colostrum gives antibodies.

The male reproductive system lies in the pelvic region and consists of a pair of testes, accessory ducts, glands and the external genitalia. The testes sit outside the abdominal cavity in a pouch, the scrotum, which keeps them 2–2.5 °C below normal internal body temperature — necessary for spermatogenesis. Each adult testis is oval, about 4–5 cm long and 2–3 cm wide, with a dense covering, and has about 250 compartments called testicular lobules. Each lobule contains one to three highly coiled seminiferous tubules, where sperms are made — so a testis has very roughly 250–750 tubules (our arithmetic).

Each seminiferous tubule is lined inside by two kinds of cells: male germ cells (spermatogonia), which undergo meiosis and finally form sperms, and Sertoli cells, which nourish the germ cells. The spaces outside the tubules — the interstitial spaces — contain small blood vessels and interstitial or Leydig cells, which make and secrete the testicular hormones, androgens; other immunologically competent cells are also present.

The male accessory ducts are the rete testis, vasa efferentia, epididymis and vas deferens. Seminiferous tubules open through the rete testis into the vasa efferentia, which leave the testis and open into the epididymis along its back surface. The epididymis leads to the vas deferens, which rises into the abdomen, loops over the urinary bladder, receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct. These ducts store and carry sperms to the outside through the urethra, which runs from the bladder through the penis to its opening, the urethral meatus. The penis is made of special tissue that allows erection for insemination; its enlarged end, the glans penis, is covered by a loose fold of skin, the foreskin.

The male accessory glands are the paired seminal vesicles, a single prostate and the paired bulbourethral glands. Their secretions form the seminal plasma, rich in fructose, calcium and certain enzymes; bulbourethral secretions also lubricate the penis.

The female reproductive system consists of a pair of ovaries, a pair of oviducts, the uterus, cervix, vagina and the external genitalia in the pelvic region; with a pair of mammary glands, they work together to support ovulation, fertilisation, pregnancy, birth and child care. Ovaries are the primary female sex organs, producing the female gamete (ovum) and several steroid (ovarian) hormones. Each ovary lies on one side of the lower abdomen, is about 2–4 cm long, and is held to the pelvic wall and uterus by ligaments. A thin epithelium covers the ovarian stroma, which has an outer cortex and an inner medulla.

The oviducts (fallopian tubes), uterus and vagina are the female accessory ducts. Each fallopian tube is about 10–12 cm long and runs from the edge of an ovary to the uterus. Nearest the ovary is the funnel-shaped infundibulum, whose finger-like fimbriae help collect the ovum after ovulation; it leads to the wider ampulla; the last part, the isthmus, has a narrow lumen and joins the uterus. The uterus (womb) is single, shaped like an inverted pear, and held by ligaments; it opens into the vagina through a narrow cervix, whose cavity, the cervical canal, forms the birth canal with the vagina. Its wall has three layers: the thin outer perimetrium, the thick middle myometrium of smooth muscle (which contracts strongly during delivery), and the inner glandular endometrium lining the cavity, which changes cyclically during the menstrual cycle.

The external genitalia are the mons pubis (a cushion of fatty tissue covered by skin and pubic hair), the labia majora (fleshy folds surrounding the vaginal opening), the labia minora (paired folds under them), the hymen and the clitoris (a small structure at the upper junction of the labia minora, above the urethral opening). The vaginal opening is often partly covered by a membrane, the hymen. NCERT states clearly that the hymen may be torn by a fall or jolt, tampon use, or sports such as horse riding and cycling, that it may persist after intercourse, and that its presence or absence is not a reliable indicator of virginity or sexual experience.

Mammary glands. A functional mammary gland is characteristic of all female mammals. The paired breasts contain glandular tissue and a variable amount of fat. The glandular tissue of each breast has 15–20 mammary lobes containing clusters of cells called alveoli, which secrete milk stored in their cavities.

The testis and ovary make gametes by gametogenesis. In the testis, spermatogenesis begins at puberty. Spermatogonia on the inside wall of the seminiferous tubules multiply by mitosis; each is diploid with 46 chromosomes. Some of them — the primary spermatocytes — periodically undergo meiosis. A primary spermatocyte completes the first meiotic (reduction) division, giving two equal, haploid secondary spermatocytes with 23 chromosomes each. These undergo the second meiotic division to give four equal, haploid spermatids. Spermatids are transformed into spermatozoa (sperms) by spermiogenesis; the sperm heads become embedded in Sertoli cells and are finally released from the tubules by spermiation.

Hormonal control. Spermatogenesis starts at puberty because secretion of gonadotropin-releasing hormone (GnRH), a hypothalamic hormone, rises sharply. GnRH acts on the anterior pituitary, which secretes two gonadotropins: luteinising hormone (LH) and follicle-stimulating hormone (FSH). LH acts on Leydig cells, stimulating androgen synthesis; androgens in turn stimulate spermatogenesis. FSH acts on Sertoli cells, stimulating factors that help spermiogenesis.

The sperm. A sperm is microscopic and has a head, neck, middle piece and tail, all covered by a plasma membrane. The head holds an elongated haploid nucleus, its front capped by the acrosome, filled with enzymes that help fertilise the ovum. The middle piece holds many mitochondria, which produce energy for the tail's movement — the motility needed for fertilisation. A man ejaculates about 200–300 million sperms per coitus; for normal fertility, NCERT says, at least 60 per cent must have normal shape and size and at least 40 per cent must show vigorous motility. Secretions of the epididymis, vas deferens, seminal vesicle and prostate are essential for sperm maturation and motility. Sperms plus seminal plasma make up the semen; androgens maintain the functions of the male accessory ducts and glands.

Oogenesis — forming a mature female gamete — is markedly different from spermatogenesis. It begins during embryonic development: a couple of million gamete mother cells, oogonia, form in each fetal ovary, and no more are formed after birth. They begin dividing, enter prophase I of meiosis and are arrested there as primary oocytes. Each primary oocyte is surrounded by a layer of granulosa cells — a primary follicle. Many follicles degenerate between birth and puberty, so only 60,000–80,000 primary follicles are left in each ovary at puberty.

Primary follicles gain more granulosa layers and a new theca to become secondary follicles, which soon become tertiary follicles, marked by a fluid-filled cavity, the antrum; the theca divides into an inner theca interna and outer theca externa. At this stage the primary oocyte grows and completes its first meiotic division — an unequal division giving a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte keeps most of the nutrient-rich cytoplasm. Whether the first polar body divides further or degenerates, NCERT says, is not certain. The tertiary follicle becomes the mature Graafian follicle; the secondary oocyte forms a new membrane, the zona pellucida; and the Graafian follicle ruptures to release the secondary oocyte (ovum) — ovulation.

The reproductive cycle of female primates — monkeys, apes and humans — is the menstrual cycle. The first menstruation, at puberty, is menarche. In humans menstruation repeats at an average interval of about 28/29 days; the events from one menstruation to the next form the cycle, and one ovum is released in the middle of each cycle.

The cycle begins with the menstrual phase, when the flow occurs for 3–5 days: the endometrial lining of the uterus and its blood vessels break down, forming a liquid that leaves through the vagina. Menstruation happens only if the released ovum is not fertilised; its absence may indicate pregnancy, but can also have other causes such as stress or poor health. In the follicular phase that follows, primary follicles grow into a mature Graafian follicle and the endometrium regenerates by proliferation, driven by pituitary and ovarian hormones. Secretion of the gonadotropins LH and FSH rises gradually, stimulating follicle growth and estrogen secretion by the growing follicles. Both LH and FSH peak in the middle of the cycle (about the 14th day); the rapid rise of LH to its maximum — the LH surge — makes the Graafian follicle rupture and release the ovum: ovulation (the ovulatory phase).

In the luteal phase that follows, the remains of the Graafian follicle become the corpus luteum, which secretes large amounts of progesterone, essential for maintaining the endometrium — necessary for implantation and the rest of pregnancy. During pregnancy all events of the cycle stop and there is no menstruation. Without fertilisation the corpus luteum degenerates, the endometrium disintegrates and menstruation starts a new cycle. Cycles cease around 50 years of age — menopause. Regular cycles indicate a normal reproductive phase, from menarche to menopause.

During coitus, semen is released by the penis into the vagina — insemination. Motile sperms swim through the cervix, into the uterus and finally reach the ampullary region of the fallopian tube. The ovum released by the ovary is also carried to the ampulla, where fertilisation takes place. Fertilisation can occur only if ovum and sperms reach the ampulla at the same time, which is why not every coitus leads to pregnancy.

During fertilisation a sperm contacts the zona pellucida and induces changes in the membrane that block the entry of further sperms, so only one sperm fertilises an ovum. The acrosome's secretions help the sperm through the zona pellucida and plasma membrane into the ovum's cytoplasm. This triggers the completion of the second meiotic division of the secondary oocyte — also unequal — giving a second polar body and a haploid ovum (ootid). The haploid nuclei of sperm and ovum then fuse to form a diploid zygote with 23 + 23 = 46 chromosomes.

Sex determination. The sex of the baby is decided at fertilisation. The chromosome pattern in the human female is XX and in the male XY, so every ovum carries an X, while sperms carry either X or Y — 50 per cent each. An X-bearing sperm gives an XX zygote, which develops into a girl; a Y-bearing sperm gives an XY zygote, a boy. In NCERT's words, it is scientifically correct to say that the sex of the baby is determined by the father, not the mother.

Cleavage — mitotic division — starts as the zygote moves through the isthmus of the oviduct towards the uterus, giving 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is a morula. The morula keeps dividing and becomes a blastocyst as it moves further into the uterus. In the blastocyst the blastomeres form an outer layer, the trophoblast, and an inner group attached to it, the inner cell mass. The trophoblast attaches to the endometrium and the inner cell mass differentiates as the embryo. After attachment the uterine cells divide rapidly and cover the blastocyst, which becomes embedded in the endometrium — implantation, which leads to pregnancy.

NCERT adds that the inner cell mass contains cells called stem cells, which have the potency to give rise to all tissues and organs.

After implantation, finger-like projections called chorionic villi appear on the trophoblast, surrounded by uterine tissue and maternal blood. Villi and uterine tissue interlock to form the placenta, a structural and functional unit between the embryo (foetus) and the mother. It supplies oxygen and nutrients to the embryo and removes carbon dioxide and wastes; it is linked to the embryo by the umbilical cord. The placenta is also an endocrine tissue, producing human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens and progestogens. Late in pregnancy the ovary also secretes relaxin. NCERT notes that hCG, hPL and relaxin are produced only during pregnancy, while the levels of estrogens, progestogens, cortisol, prolactin, thyroxine and others rise several-fold — needed for fetal growth, the mother's metabolic changes and maintenance of pregnancy.

Immediately after implantation the inner cell mass differentiates into an outer ectoderm and an inner endoderm; a mesoderm soon appears between them. These three layers give rise to all the tissues and organs of the adult. Human pregnancy lasts about 9 months. After one month the embryo's heart is formed; the first sign of a growing foetus can be noticed by listening for its heart sound through a stethoscope. By the end of the second month the foetus develops limbs and digits. By the end of 12 weeks (the first trimester) most major organ systems are formed — limbs and external genital organs are well developed. The first movements of the foetus and hair on the head usually appear in the fifth month. By about 24 weeks (end of the second trimester) the body is covered with fine hair, the eyelids separate and eyelashes form. By the end of nine months the foetus is fully developed and ready for delivery.

The average human pregnancy — the gestation period — is about 9 months. At its end, vigorous contraction of the uterus expels the foetus: parturition (childbirth). It is induced by a complex neuroendocrine mechanism. Signals from the fully developed foetus and placenta induce mild uterine contractions, the foetal ejection reflex, which triggers the release of oxytocin from the mother's pituitary. Oxytocin acts on uterine muscle and causes stronger contractions, which in turn stimulate more oxytocin. This reflex continues, giving stronger and stronger contractions until the baby is expelled through the birth canal. Soon after, the placenta is also expelled.

The mammary glands differentiate during pregnancy and begin producing milk towards its end — lactation, which lets the mother feed the newborn. The milk of the first few days, colostrum, contains several antibodies essential for the newborn's resistance. Doctors recommend breastfeeding during the early period of infant growth for a healthy baby.

The chapter asks questions in italics inside the running text. Short outlines follow, in page order.

Try each question first, then open the outline. For diagram questions, draw neatly and label every part named in the text.

Numbers

One-liners

• Sertoli cells nourish germ cells; Leydig cells secrete androgens.
• LH acts on Leydig cells, FSH on Sertoli cells.
• Seminal plasma is rich in fructose, calcium and enzymes.
• Spermiogenesis turns spermatids into sperms; spermiation releases them.
• The acrosome holds enzymes; the middle piece holds mitochondria.
• Oogenesis starts before birth; primary oocytes wait in prophase I.
• The antrum first appears in the tertiary follicle.
• The LH surge causes ovulation; the corpus luteum secretes progesterone.
• Fertilisation occurs in the ampulla of the fallopian tube.
• Meiosis II of the oocyte finishes only after sperm entry.
• The sperm (X or Y) decides the baby's sex.
• Trophoblast → placenta; inner cell mass → embryo and stem cells.
• hCG, hPL and relaxin are made only in pregnancy (NCERT).
• Oxytocin drives a positive-feedback loop in parturition.
• Colostrum carries antibodies to the newborn.

Traps to avoid

• Androgens come from Leydig cells, not Sertoli cells.
• Secondary spermatocytes are already haploid (23); reduction happens in meiosis I.
• The cell released at ovulation is a secondary oocyte; NCERT also calls it the ovum.
• Oogenesis gives one ovum per primary oocyte; spermatogenesis gives four sperms.
• Follicular = proliferative phase; luteal = secretory phase.
• Menstruation follows the corpus luteum's degeneration, not an LH surge.
• Fertilisation is in the ampulla, implantation in the endometrium of the uterus.
• Identical twins: one egg; fraternal twins: two eggs.
• Cleavage is mitosis; morula is 8–16 cells.
• The placenta is partly fetal (chorionic villi) and partly maternal (uterine tissue).
• NCERT's text names only oxytocin for parturition; its Summary adds cortisol and estrogens.
• PCPNDT bans sex selection, not prenatal diagnosis as such.

Prelims — what UPSC asks

• Where and who: fertilisation in the fallopian tube (2007); the corpus luteum in the ovary (1997); sex determined by the father's chromosomes (1984) and at fertilisation (1981); the assertion that females determine the sex is false though women do have two X chromosomes (2000); identical twins from one zygote that splits (1981). All follow directly from Sections 4–7.
• Disputed or flawed keys to watch: 1994 asked when the embryo's heart starts beating; the stored key is ‘third week’. Embryology texts place the first beats at about day 21–22 after fertilisation — the end of the third or start of the fourth week — so the key is disputed between (b) and (c), and NCERT itself says only that the heart is formed after one month. 1981 ‘sex of the offspring is determined when’ is a flawed question: options (a) ‘conception takes place’ and (b) ‘the sperm fertilises the ovum’ describe the same event; the stored key is (b).
• Likely next: Sertoli vs Leydig cells, LH and FSH targets, hormones of pregnancy (hCG in pregnancy tests), the menstrual phases, the placenta, stem cells from the inner cell mass, and the laws in Section 10 (asked as current affairs).

Mains — where it appears and how to answer

• No GS Mains question in the bank so far is built on this chapter's biology itself. Its use is as the scientific base for: GS1 — son preference and the child sex ratio (start from ‘the sperm decides’); GS2 — maternal and child health, menstrual hygiene, breastfeeding and maternity benefits, the PCPNDT, MTP, ART and Surrogacy Acts; GS3 — stem-cell technology and the ethics of assisted reproduction.
• Answer frame for a reproductive-health or regulation question: one line of biology (e.g. how sex is determined, or what IVF involves) → the social problem (son preference, unsafe abortion, exploitation) → the law and scheme → positions on contested points, attributed → implementation gaps → balanced way forward.
• Use respectful, clinical language throughout; a small labelled sketch (menstrual-cycle timeline, sex-determination cross) can make a point quickly.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

Open any question to attempt it in the Question bank — your attempt is saved to your progress.