• In placental mammals the ovary becomes active at definite intervals (the breeding/sexual cycle) that coincide with mating.
• The length differs once a year (lion, tiger), twice a year (cat, dog) or several times a month (rabbit, rat).
• In women it is the menstrual cycle, usually 28 days, in which the two ovaries alternate in producing a mature ovum.
• Phases of the cycle (counting day 1 as the first day of bleeding):
• (a) Menstruation (days 1–5) the endometrium breaks down and is shed with blood.
• (b) Proliferation (follicular) phase (days ~5–13):

• the pituitary releases FSH, which stimulates the ovary to develop several follicles, of which one becomes the Graafian follicle (the ovum matures in about 10 days).
• The follicle secretes oestrogen, which stimulates the endometrium to grow (proliferate) and become thick and vascular.
• High oestrogen then stimulates a surge of LH.
• (c) Ovulation (day 14):

• the pituitary releases a surge of LH, which causes the Graafian follicle to rupture and release the secondary oocyte (the ovum with the first polar body).
• The remains of the follicle turn into the corpus luteum (yellow body), which secretes progesterone and oestrogen.
• (d) Secretory (luteal) phase (days 15–28):

• progesterone and oestrogen from the corpus luteum further thicken the endometrium, increase its blood supply and glands so that it is ready for the embryo.
• These hormones also inhibit FSH and LH (negative feedback), so no new follicle develops.
• (e) If the ovum is not fertilised the corpus luteum degenerates gradually; progesterone and oestrogen fall; the endometrium breaks down; the blood vessels rupture from uterine contractions; menstrual bleeding follows (about 3–5 days) and a new cycle (with the other ovary) begins.
• (f) If the ovum is fertilised:

• the corpus luteum persists and continues secreting oestrogen and progesterone, which maintain the endometrium and inhibit ovulation, so the cycle stops until after birth.
• The corpus luteum is at its maximum at the end of the third month and starts to degenerate in the fourth month, when the placenta takes over secreting progesterone and oestrogen and preserves the endometrium.
• Progesterone also gradually develops the mammary glands.
• If the corpus luteum is removed before the fourth month, miscarriage (abortion) occurs; after that, it does not, because the placenta supplies the hormones.

✍️ Worked examples

• Worked example 12 (cycle length).
• For a woman with a 32-day cycle, ovulation happens about 14 days before the next menstruation, i.e.
• on day 18; the luteal phase remains about 14 days, so only the follicular phase is lengthened.
• Worked example 13. A woman on the contraceptive pill never ovulates. Why? The pill contains synthetic oestrogen and progesterone, which keep the blood level high, inhibiting FSH and LH from the pituitary → no follicle ripens and no ovulation.

• Usually one baby per birth; sometimes multiple births (up to six). The most common are twins — about 1 in 86 births worldwide.
• (a) Fraternal (dizygotic, non-identical) twins:

• two ova are released together (from one or both ovaries) and fertilised by two sperms; each develops as an independent embryo with its own amnion/sac and placenta.
• They differ genetically (as ordinary brothers and sisters), may be of different sex.
• (b) Identical (monozygotic) twins:

• one ovum fertilised by one sperm; the dividing zygote (in the early cleavage or blastocyst stage) separates into two masses of cells, each developing as an embryo.
• They are genetically identical (same sex, same characteristics), usually share one placenta and may share the same amniotic sac.
• Incomplete separation gives conjoined (Siamese) twins, which can often be separated surgically.

1. Scientists’ efforts in reproductive technology help many families
2. but they must be used within moral and religious rules
3. with respect for human life and the Great Creator’s system of reproduction.

• 📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

• DABOUR HUB ACADEMY • EGYPTIAN HIGH SCHOOL (THANAWEYA AMMA)
• EXPLANATION Lesson 4: Menstrual Cycle, Fertilization, Embryonic Development and Contraception
• Course Biology (Senior 3) | Chapter 3 – Reproduction in Living Organisms | Official Ministerial Syllabus 2026/2027
• ―――――――――――――――――――――――――――――――――――――――――――――――――――――――

1. Academic Lesson Overview & Pedagogical Objectives

1. The hormonal regulation of the 28-day menstrual cycle
2. the journey of fertilization in the Fallopian tube
3. twin formation (identical vs. fraternal)
4. and modern contraceptive methods.

2. Hierarchy Classification Tree (Cognitive Concept Map)

• Below is the hierarchical structural tree organizing the conceptual core of this lesson:

Cycle, Pregnancy & Contraception

├── 1. Menstrual Cycle (28 Days)

• │ ├── Proliferation Phase (Days 5-14) FSH stimulates Graafian follicle → Estrogen builds endometrium
• │ ├── Ovulation Phase (Days 14-28) LH surge triggers ovum release; Corpus Luteum secretes Progesterone
• │ └── Menstruation Phase (Days 1-5) If no fertilization, Corpus Luteum degenerates, Progesterone drops, bleeding occurs
• ├── 2. Fertilization & Cleavage: Morula (day 7) implants into uterine endometrium; Placenta & Umbilical cord
• ├── 3. Multi-Fetal Pregnancy: Identical Twins (Monozygotic: 1 egg + 1 sperm) vs. Fraternal Twins (Dizygotic: 2 eggs + 2 sperm)
• └── 4.
• Contraception: Pills (Inhibit ovulation), IUD (Prevents implantation), Condom (Prevents sperm entry), Surgical Sterilization (Permanent)

3. The Master Teacher's Blackboard & Exam Rules