Reproductive Biology

8.1 Modes of Reproduction — Asexual to Sexual

Reproduction is the biological process by which organisms produce new individuals of the same species, ensuring continuity of life. All reproductive strategies fall into two broad categories: asexual (without gamete fusion; offspring genetically identical to parent) and sexual (involving meiosis, gametes, and syngamy; offspring genetically varied).

8.1.1 Asexual Reproduction

Asexual reproduction is common in simpler organisms and produces genetically identical progeny (clones). Key types include:

• Binary fission — parent divides equally into two daughters. Occurs in Amoeba, Paramecium, bacteria. In Plasmodium (malarial parasite) the process is multiple fission (schizogony): one trophozoite divides simultaneously into many merozoites inside the RBC.
• Budding — small outgrowth (bud) differentiates and detaches. Examples: Hydra (somatic budding), yeast (Saccharomyces). Sponges form internal buds called gemmules.
• Fragmentation — body breaks into pieces, each regenerating. Spirogyra (filamentous alga), Planaria (flatworm). Distinct from regeneration in that fragmentation is the normal reproductive mode.
• Regeneration — regrowth of lost body parts from remaining tissue. Hydra and Planaria show high regenerative capacity; famously studied by Abraham Trembley (1740) in Hydra.
• Sporulation — formation of asexual spores resistant to unfavourable conditions. Common in fungi (Rhizopus) and protists.
• Parthenogenesis — development of an unfertilised egg into a new individual. Haploid drones (male honeybees) develop by parthenogenesis from unfertilised eggs; also seen in some lizards (Cnemidophorus), aphids, Daphnia, and turkey. Artificially induced parthenogenesis is possible in frogs.
• Vegetative propagation — in plants via runners, rhizomes, bulbs, tubers, leaves, etc. (covered in Ch. 1 §1.9). Agave produces bulbils; Bryophyllum regenerates from leaf margins.

8.1.2 Sexual Reproduction — Overview

Sexual reproduction involves three essential phases: pre-fertilisation (gametogenesis and gamete transfer), fertilisation (syngamy — fusion of haploid gametes to restore 2n), and post-fertilisation (zygote development, embryogenesis). Based on gamete morphology, fertilisation is classified as:

• Isogamy — fusion of morphologically identical (isogametes) gametes; seen in some algae (Chlamydomonas).
• Anisogamy — gametes differ in size but both motile; Chlamydomonas species.
• Oogamy — large non-motile egg + small motile sperm; the rule in all animals, most higher plants. Highly advanced form of sexual reproduction.

8.2 Human Male Reproductive System & Spermatogenesis

The male reproductive system produces, stores, and delivers spermatozoa, and secretes the hormone testosterone. Its components are the paired testes, accessory ducts, glands, and the penis.

8.2.1 Gross Anatomy

Testes are paired oval organs (~4 cm × 2.5 cm) suspended in the scrotum outside the abdominal cavity. The scrotal temperature is 2–3°C below core body temperature — essential for spermatogenesis because sperm production fails at body temperature. Each testis is divided by fibrous septa into ~250 testicular lobules, each containing 1–3 highly coiled seminiferous tubules (total ~250 m when uncoiled). The interstitial space between tubules houses Leydig cells (interstitial cells) which secrete testosterone in response to LH.

Each seminiferous tubule is lined by two cell types: Sertoli cells (large nurse cells; form the blood–testis barrier, BTB; support spermatogenic cells; secrete inhibin and ABP) and spermatogenic cells (spermatogonia → primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa).

Accessory ducts: Rete testis → efferent ductules → epididymis (site of sperm maturation and storage; 6 m long, highly coiled) → vas deferens (ductus deferens; cut in vasectomy) → ejaculatory duct → urethra (shared with urinary system). The path can be remembered as: SEVEN = Seminiferous tubule, Epididymis, Vas deferens, Ejaculatory duct, urethra, Nothing else (penis).

Accessory glands:

• Seminal vesicles (2) — produce ~60% of semen volume; rich in fructose (energy source for sperm), prostaglandins, fibrinogen.
• Prostate gland (1) — ~30% of semen; alkaline secretion neutralises vaginal acidity, activating sperm; contains zinc, citrate, PSA.
• Bulbourethral glands (Cowper's glands, 2) — pre-ejaculatory fluid; lubricates urethra; alkaline.

Semen = sperm + secretions of all glands. Normal semen volume 2–5 mL, pH 7.2–8.0, sperm count ≥15 million/mL (WHO 2010). Sperm count <15 M/mL = oligospermia; absence = azoospermia.

8.2.2 Sperm Structure

A human spermatozoon is ~60 μm long and consists of four regions:

• Head — contains the haploid (n) nucleus with tightly coiled chromatin; capped by the acrosome (derived from the Golgi apparatus during spermiogenesis), which is a lysosome-like vesicle packed with hydrolytic enzymes (hyaluronidase, acrosin) needed to penetrate the zona pellucida during fertilisation.
• Neck — short; contains a pair of centrioles (proximal centriole initiates first cleavage after fertilisation; distal centriole gives rise to the axoneme).
• Middle piece — densely packed with mitochondria arranged in a helical sheath; provides ATP for flagellar movement. The energy for sperm motility comes from mitochondrial respiration (fructose substrate from seminal vesicle).
• Tail (flagellum) — 9+2 axoneme microtubule arrangement (9 outer doublets + 2 central singlets); dynein ATPase provides sliding force. The tail propels sperm at ~1–4 mm/min in the female tract.

8.2.3 Spermatogenesis

Spermatogenesis is the process by which diploid spermatogonia give rise to haploid spermatozoa. It begins at puberty, continues throughout adult life, and takes ~74 days in humans. It occurs in the seminiferous tubules and is divided into three phases:

1. Multiplication phase: Type A spermatogonia (2n) undergo repeated mitoses to proliferate. Some remain as stem cells; others differentiate into Type B spermatogonia, which then become primary spermatocytes (2n, 4C after DNA replication).
2. Growth phase: Primary spermatocytes grow in size; DNA replication (S phase) has already occurred, so each contains 4C DNA.
3. Maturation phase (meiosis):
   • Meiosis I: Primary spermatocyte (2n, 4C) → two secondary spermatocytes (n, 2C). This is the reduction division. Takes ~22 days; cells arrested briefly at the end of prophase I.
   • Meiosis II: Each secondary spermatocyte (n, 2C) → two spermatids (n, 1C). This is the equational division.
   • Net: 1 primary spermatocyte → 4 spermatids (all functional; all haploid).

Spermiogenesis is the transformation of non-motile, round spermatids into streamlined spermatozoa: chromatin condenses; acrosome forms from Golgi; flagellum develops from distal centriole; cytoplasm is shed as residual body (phagocytosed by Sertoli cells). Spermiogenesis is not a cell division.

Spermiation is the release of mature spermatozoa from Sertoli cells into the tubule lumen.

8.2.4 Hormonal Regulation of Spermatogenesis

The hypothalamo–pituitary–gonadal (HPG) axis controls spermatogenesis:

• Hypothalamus secretes GnRH (gonadotrophin-releasing hormone) in pulses → anterior pituitary.
• Anterior pituitary secretes FSH (stimulates Sertoli cells; promotes spermatogenesis) and LH (stimulates Leydig cells to secrete testosterone).
• Testosterone is required for spermatogenesis, development of secondary sexual characters, maintenance of accessory glands, and libido. It also exerts negative feedback on the hypothalamus and pituitary.
• Inhibin is secreted by Sertoli cells and selectively inhibits FSH release (negative feedback).
• Androgen-binding protein (ABP), secreted by Sertoli cells, concentrates testosterone in the tubule lumen to levels needed for spermatogenesis.

Regnier de Graaf — first described ovarian follicles 1672 (the “Graafian follicle” bears his name) · Lazzaro Spallanzani — first artificial insemination (dog) 1779; demonstrated sperm necessity · Carl Hartman — demonstrated ovulation timing in macaques 1929 · Edwards & Steptoe — first IVF baby (Louise Brown) 28 July 1978; Edwards Nobel Prize 2010 · Pincus & Chang — oral contraceptive pill clinical trials 1956; first marketed 1960 (Enovid) · Carl Djerassi — synthesised norethindrone (first oral progestogen) 1951

8.3 Human Female Reproductive System & Oogenesis

8.3.1 Gross Anatomy

Ovaries are paired almond-shaped organs (~3 cm × 1.5 cm) in the pelvic cavity, held by ligaments (ovarian ligament, suspensory ligament). Each ovary has a cortex (contains follicles at all stages) and a medulla (blood vessels, nerves). Unlike testes, ovaries do not descend.

Fallopian tubes (oviducts) are paired, ~10–12 cm long, connecting ovary to uterus. Regions from ovary to uterus: infundibulum (funnel; bears finger-like fimbriae which sweep the ovulated oocyte into the tube), ampulla (widest; site of fertilisation), isthmus (narrow; connects to uterus). Cilia and peristalsis move the oocyte/embryo toward the uterus. If fertilisation occurs in the ampulla, the zygote reaches the uterus ~3–4 days later.

Uterus (womb) — single pear-shaped muscular organ, ~7.5 cm long, lying in the pelvis between bladder and rectum. Layers:

• Perimetrium (outer serosa, from peritoneum)
• Myometrium (thick smooth muscle; contracts during labour and menstruation)
• Endometrium (inner mucosa; undergoes cyclic changes; the functional layer is shed at menstruation; the basal layer regenerates each cycle)

The lower narrow end is the cervix (opening = os uteri); cervical mucus consistency changes with cycle phase — it is thin and watery around ovulation (facilitates sperm entry) and thick/viscous during the luteal phase (barrier to sperm). Below the cervix is the vagina.

External genitalia (vulva): Mons pubis, labia majora, labia minora, clitoris (homologous to penis), vestibule with urethral opening and vaginal opening, Bartholin's (greater vestibular) glands (homologous to Cowper's glands).

8.3.2 Oogenesis

Oogenesis is the production of female gametes (ova). Unlike spermatogenesis (continuous from puberty), oogenesis begins in the foetal ovary and involves prolonged arrests.

• Foetal period: Primordial germ cells differentiate into oogonia (2n) and multiply by mitosis. By ~20 weeks gestation, ~7 million oogonia are present. These enter meiosis I and progress to diplotene stage of prophase I, where they arrest as primary oocytes. By birth, ~2 million primary oocytes remain (the rest undergo atresia). By puberty, only ~400,000 remain. A primary oocyte may stay arrested in diplotene for 12–50 years! (Termed “dictyate stage.”)
• Puberty onwards: Each month, FSH stimulates a cohort of primordial follicles; one dominant Graafian follicle is selected. The primary oocyte within it resumes and completes meiosis I, producing the secondary oocyte (n, 2C) and the first polar body (n, 2C; may or may not undergo meiosis II — ultimately degenerates). The secondary oocyte immediately enters meiosis II and arrests at metaphase II.
• Ovulation: The secondary oocyte (still in metaphase II arrest) is released from the Graafian follicle at ovulation (~day 14).
• Fertilisation: Only if a sperm penetrates the secondary oocyte does meiosis II complete, yielding the ovum (n, 1C) + the second polar body (n, 1C). The ovum nucleus and sperm nucleus then fuse to form the zygote (2n).
• Net: 1 primary oocyte → 1 functional ovum + 3 polar bodies (all degenerate). Only ~400 oocytes ovulate in a lifetime (one per cycle for ~35 years).

8.3.3 Folliculogenesis — Graafian Follicle Development

Each primary oocyte is initially surrounded by a single layer of squamous cells to form a primordial follicle. Under FSH stimulation, follicles progress through:

• Primary follicle — oocyte surrounded by a single layer of cuboidal granulosa cells; zona pellucida (glycoprotein coat, ZP1, ZP2, ZP3) forms between oocyte and granulosa.
• Secondary follicle — multiple layers of granulosa cells; theca interna and theca externa begin to differentiate outside granulosa. Theca interna secretes androgens; granulosa aromatises them to oestrogens (two-cell theory).
• Tertiary (antral) follicle — fluid-filled antrum appears between granulosa cells (antral fluid = follicular fluid; contains oestrogens, FSH, LH, inhibin, growth factors).
• Graafian (mature) follicle — large (~20 mm); cumulus oophorus (stalk of granulosa cells) holds oocyte centrally; corona radiata (innermost granulosa layer directly on oocyte). The LH surge triggers ovulation: follicle wall ruptures, secondary oocyte + corona radiata are expelled.
• Corpus luteum — ruptured follicle filled with blood (corpus haemorrhagicum) → granulosa and theca cells luteinise (become corpus luteum) and secrete progesterone (major) + oestrogen. If pregnancy: hCG from trophoblast maintains corpus luteum; if no pregnancy: corpus luteum degenerates (~day 24) → corpus albicans (fibrous scar).

8.4 Menstrual Cycle & Reproductive Endocrinology

The menstrual cycle is the ~28-day (range 21–35 days) recurring cycle of endometrial and hormonal changes in a sexually mature, non-pregnant woman. It begins at menarche (first menstruation; ~12–13 years) and ends at menopause (~45–55 years, average 51 years). The cycle prepares the uterus for potential implantation and, if fertilisation does not occur, sheds the prepared endometrium.

8.4.1 Phases of the Menstrual Cycle

8.4.2 Hormonal Dynamics

The cycle is driven by the HPG axis with two-way feedback:

• Follicular phase: Low oestrogen exerts negative feedback on the pituitary, keeping LH low. Rising FSH stimulates follicle growth; granulosa cells secrete increasing oestrogen. Inhibin B (from granulosa) suppresses FSH toward mid-follicular phase, ensuring selection of a single dominant follicle.
• Pre-ovulatory switch: When oestrogen rises above a threshold (~200 pg/mL for ≥50 hours), it switches to positive feedback, triggering the LH surge (and smaller FSH surge) ~36 hours before ovulation. The LH surge is the trigger for ovulation (follicle rupture) and luteinisation.
• Luteal phase: Corpus luteum secretes progesterone (peaks day 21–22) + oestrogen. Progesterone exerts negative feedback on GnRH/LH, preventing a new LH surge. If no pregnancy, corpus luteum regresses (luteolysis), progesterone falls → endometrial arteries spasm → functional layer ischaemia → menstruation. The fall in progesterone releases the HPG axis, and a new cycle begins.
• If pregnancy: Trophoblast secretes hCG (human chorionic gonadotrophin) by ~8 days post-fertilisation; hCG has LH-like activity, rescuing the corpus luteum — its progesterone production maintains the endometrium. This is the basis of pregnancy tests (detect hCG in urine/blood).

8.5 Fertilisation, Implantation & Embryonic Development

8.5.1 Fertilisation

Fertilisation normally occurs in the ampulla of the fallopian tube. Sperm deposited in the vagina must reach the ampulla — a journey of ~15–18 cm against cervical mucus, through the uterine cavity and into the tube. Only ~200 sperm (of ~200 million ejaculated) reach the vicinity of the oocyte; the rest are lost to acidity, phagocytosis, or misdirection.

Steps of fertilisation:

1. Capacitation: Sperm must undergo physiological maturation in the female tract (~7 hours); the plasma membrane overlying the acrosome becomes destabilised, cholesterol is removed, and sperm become hyperactivated (whiplash motility).
2. Acrosomal reaction: On contact with the zona pellucida, ZP3 (zona pellucida glycoprotein 3) binds to sperm receptors, triggering exocytosis of acrosomal contents: hyaluronidase (disperses corona radiata hyaluronic acid matrix) and acrosin (serine protease that digests ZP locally). The sperm bores through zona pellucida.
3. Sperm–oocyte membrane fusion: Sperm plasma membrane fuses with oocyte plasma membrane; the sperm head (nucleus + centrioles) enters the egg cytoplasm. The sperm tail is usually lost.
4. Cortical reaction (block to polyspermy): Within seconds, cortical granules (lysosome-like; just under oocyte plasma membrane) exocytose their contents into the perivitelline space. Enzymes modify ZP2 and ZP3, hardening the zona pellucida into the impenetrable fertilisation membrane. This prevents entry of additional sperm (polyspermy prevention). A fast block also occurs instantly via membrane depolarisation.
5. Resumption of meiosis II: Sperm entry provides the signal (Ca²⁺ wave) to complete meiosis II: secondary oocyte → ovum (n) + 2nd polar body.
6. Pronuclei & syngamy: Sperm nucleus decondenses to form the male pronucleus; egg nucleus forms the female pronucleus. The two pronuclei approach each other, nuclear envelopes break down, and chromosomes unite on the first cleavage spindle. This fusion is syngamy; the resulting cell is the zygote (2n = 46 chromosomes).

8.5.2 Cleavage and Morula

After fertilisation, the zygote undergoes rapid mitotic divisions called cleavage without growth — the cells (blastomeres) halve in size with each division. In humans, cleavage is holoblastic and equal (yolk is minimal; all blastomeres are equal). Key stages:

• 2-cell stage — ~30 hours post-fertilisation
• 4-cell — ~40 hours
• 8-cell — ~72 hours; compaction occurs (blastomeres flatten and adhere tightly via E-cadherin)
• Morula (16–32 cells, solid ball) — ~4 days; still surrounded by zona pellucida; travelling down the fallopian tube
• Blastocyst (32–64 cells) — ~5 days; fluid accumulates (blastocoele); two distinct cell populations appear:
  • Inner cell mass (ICM / embryoblast) — cluster of cells that will form the embryo and all embryonic membranes
  • Trophoblast — outer layer; will form the placenta and chorion
• Hatching — blastocyst escapes the zona pellucida (~day 5–6) by enzymatic digestion; must hatch before implantation can occur.

8.5.3 Implantation

The blastocyst arrives in the uterine cavity ~4–5 days post-fertilisation and implants into the endometrium ~7–8 days post-fertilisation (day 20–22 of the menstrual cycle). Normal implantation site: posterior wall of the uterine body (upper 2/3). Abnormal implantation in the fallopian tube = ectopic pregnancy (tubal pregnancy; life-threatening).

Implantation steps:

1. Apposition — blastocyst loosely contacts the endometrium (ICM pole toward decidua).
2. Adhesion — trophoblast binds to endometrial epithelium via integrins and selectins.
3. Invasion — trophoblast differentiates into syncytiotrophoblast (invasive, multinucleate; invades endometrial stroma and erodes maternal blood vessels) and cytotrophoblast (inner stem-cell layer). Syncytiotrophoblast begins secreting hCG by day 8, detectable in blood by day 10 and in urine by day 12–14 (basis of home pregnancy tests).
4. Endometrium transforms into the decidua (decidualisation) — glycogen-rich stromal cells provide nutrition for early embryo. Three decidual regions: decidua basalis (beneath embryo; becomes maternal placenta), decidua capsularis (overlying embryo), decidua parietalis (rest of uterine cavity).

8.5.4 Embryonic Development — Key Milestones

• Week 2: Bilaminar embryonic disc (epiblast + hypoblast). Amniotic cavity and yolk sac form. hCG peaks.
• Week 3 — Gastrulation: Trilaminar embryonic disc (ectoderm, mesoderm, endoderm). Primitive streak forms. Notochord induces neural plate (neurulation begins). Most critical week — teratogen exposure highly damaging.
• Weeks 4–8 — Organogenesis: All major organ systems laid down. Heart beats by day 22 (most sensitive to teratogens). Limb buds appear week 4. By week 8, all major organs are present — the organism is called an embryo.
• Week 9 onward — Foetal period: The conceptus is now a foetus; organs grow and mature. Sex determination externally visible by week 12. Foetal movements (“quickening”) felt by mother ~18–20 weeks.
• Gestation: Human pregnancy lasts ~280 days (40 weeks, 9 calendar months) from last menstrual period (LMP), or ~266 days from fertilisation.

8.5.5 Placenta

The placenta is the disc-shaped organ (haemochorial type in humans) at the interface of maternal and foetal circulations. It is composed of:

• Foetal component: chorionic villi (extensions of trophoblast + foetal mesoderm containing foetal capillaries); fully formed by week 10–12.
• Maternal component: decidua basalis; intervillous space filled with maternal blood (maternal arteries open into it; foetal and maternal blood do NOT mix).

Functions of the placenta: nutrition (glucose, amino acids, vitamins); gas exchange (O₂ to foetus, CO₂ to mother via diffusion); excretion (urea, bilirubin); hormone production (see below); immune function (maternal IgG crosses; passive immunity to neonate).

Placental hormones:

• hCG (human chorionic gonadotrophin) — from syncytiotrophoblast; peak weeks 8–10; maintains corpus luteum first trimester; LH-like; basis of pregnancy tests. Elevated in trisomy 21 (Down syndrome) screening.
• hPL (human placental lactogen / chorionic somatomammotrophin) — anti-insulin; mobilises maternal fat; promotes foetal growth; rises throughout pregnancy.
• Progesterone — from placenta after week 8–10 (the “luteo-placental shift”; before that the corpus luteum is the source).
• Oestrogen (oestriol) — high levels from placenta; promotes uterine growth; oestriol low in Down syndrome (triple test screening).
• Relaxin — relaxes pubic symphysis and cervix for parturition.

Umbilical cord: Contains 2 umbilical arteries (carry deoxygenated blood from foetus to placenta) + 1 umbilical vein (carries oxygenated blood from placenta to foetus). Surrounded by Wharton's jelly (mucoid connective tissue). Note: umbilical arteries carry deoxygenated blood (unique — usual arteries carry oxygenated blood).

8.6 Parturition, Lactation & Population Stabilisation

8.6.1 Parturition (Childbirth)

Parturition is the process of expulsion of the fully developed foetus from the uterus, typically at ~40 weeks. It is initiated by a complex interplay of foetal, placental, and maternal hormones involving a positive feedback loop:

1. Foetal cortisol rises near term (foetal HPA axis matures), stimulating the placenta to shift progesterone production to oestrogen synthesis (prostaglandin biosynthesis increases).
2. Rising oestrogen up-regulates myometrial oxytocin receptors and prostaglandin production; myometrium becomes more excitable.
3. Prostaglandins (PGE₂, PGF₂α) stimulate uterine contractions and ripen/soften the cervix.
4. Uterine contractions press the foetal head against the cervix → Ferguson's reflex: cervical stretch → neural signals to hypothalamus → oxytocin release from posterior pituitary → stronger contractions → more cervical stretch → more oxytocin (positive feedback). This continues until delivery.
5. After delivery: oxytocin causes uterine involution; progesterone falls sharply (placenta expelled).

Stages of labour:

• Stage 1 (Dilatation): Cervix dilates from 0 to 10 cm. Longest stage (hours to days in primiparae). Membranes usually rupture.
• Stage 2 (Expulsion): Active pushing phase; foetus expelled. Usually <2 hours.
• Stage 3 (Placental): Placenta delivered within ~30 minutes of foetal birth. Uterus contracts (further oxytocin) to prevent haemorrhage.

8.6.2 Lactation

After delivery, the maternal breast undergoes milk production (lactogenesis) and milk ejection (galactokinesis):

• Prolactin (anterior pituitary) — hormone of milk synthesis. High oestrogen/progesterone during pregnancy inhibits prolactin action on the breast (despite high prolactin levels). After delivery, oestrogen/progesterone fall rapidly, unmasking prolactin action → milk synthesis begins 2–3 days post-partum. Suckling stimulates further prolactin release (neuroendocrine reflex).
• Oxytocin (posterior pituitary) — hormone of milk ejection (let-down reflex). Suckling sensory input → hypothalamus → oxytocin → myoepithelial cell contraction → milk ejection from alveoli into ducts.
• Colostrum — first milk secreted 2–3 days post-partum; yellow, thick, low fat, rich in protein and secretory IgA (provides passive immunity to newborn against pathogens). Also contains growth factors (EGF), lymphocytes, and anti-infective proteins. Superior to formula for neonatal immunity.
• Lactational amenorrhoea: High prolactin during exclusive breastfeeding suppresses GnRH → no LH/FSH surge → no ovulation. Provides natural (but unreliable after 6 months) contraception — the LAM (Lactational Amenorrhoea Method) is effective up to 6 months if breastfeeding is exclusive and menstruation has not returned.

8.6.3 Twins

8.6.4 Population Stabilisation & Reproductive Health Policy

India achieved “replacement fertility” (TFR = 2.1) nationally in 2021. The National Population Policy 2000 (NPP 2000) set targets to achieve TFR of 2.1 by 2010 (achieved a decade late). Key metrics:

• TFR (Total Fertility Rate): Average number of children born per woman over her lifetime. Replacement level = 2.1. HP TFR ~1.7 (2019–21).
• MMR (Maternal Mortality Ratio): Deaths per 100,000 live births. India ~97 (2018–20, SRS). Target <70 by 2030 (SDG).
• IMR (Infant Mortality Rate): Deaths per 1,000 live births before age 1. India ~28 (2020). HP ~22, below national average.
• Demographic transition theory: Nations move from high birth + high death (Stage I) → high birth + low death (Stage II, population explosion) → low birth + low death (Stage III, stable). India is in Stage III transition.
• MTP (Medical Termination of Pregnancy) Act 1971 (India): Legalised abortion up to 20 weeks under specific conditions; amended 2021 to allow up to 24 weeks for special categories (rape survivors, differently-abled women, minors, married women with contraceptive failure). Requires approval of 1 doctor (up to 20 weeks) or 2 doctors (20–24 weeks).

8.7 Reproductive Health, Contraception, IVF & STDs

8.7.1 Contraceptive Methods

Contraception is the deliberate prevention of pregnancy. Methods range from behavioural to surgical; they differ in mechanism, efficacy (Pearl Index = pregnancies/100 women-years), and reversibility.

8.7.2 Assisted Reproductive Techniques (ART)

8.7.3 Infertility

Infertility is the inability to conceive after 12 months of regular unprotected intercourse. Affects ~15% of couples globally. Causes:

• Male factors (~40%): Oligospermia (<15 M/mL); azoospermia (no sperm); asthenospermia (poor motility); teratospermia (abnormal morphology); varicocele (dilated spermatic vein, raises testicular temperature); obstructive azoospermia (vasectomy, infection-related blockage).
• Female factors (~40%): Anovulation (PCOS is most common cause — polycystic ovarian syndrome; high LH:FSH ratio, hyperandrogenism, irregular cycles); tubal blockage (often from PID/chlamydia); endometriosis (endometrial tissue outside uterus); uterine fibroids or anomalies.
• Combined / unexplained (~20%).

Treatment: Ovulation induction (clomiphene citrate, letrozole, gonadotrophins); intrauterine insemination (IUI); IVF; ICSI. Clomiphene acts as a selective oestrogen receptor modulator (SERM) — blocks oestrogen negative feedback on pituitary → increased FSH output → follicle stimulation.

8.7.4 Sexually Transmitted Diseases (STDs)

8.8 Quick-Reference Tables

End of Chapter 8 · Reproductive Biology. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.