Human Reproductive System: Menstrual Cycle, Fertilisation, Implantation & Placenta: quick revision, 15 bites

The Menstrual Cycle: A 28-Day Hormonal Rhythm

One cycle, four phases, one egg.

The menstrual cycle is a recurring ~28-day ovarian cycle in females, ending in menstruation. It is divided into **four phases** controlled by hormones.

Takeaway. The cycle is counted from the first day of menstrual flow to the next onset, about 28 days later.

Menarche, Menopause & Menstruation

Three M-words that define reproductive life.

Takeaway. Menstruation is absent before puberty, during pregnancy, and after menopause.

How Long Does Menstrual Flow Last?

Three to five days of flow.

Takeaway. Flow duration (3–5 days) is shorter than the full cycle (28 days) — don't confuse the two.

Hormonal Control of the Menstrual Cycle

FSH, LH, estrogen, progesterone — the four conductors.

Takeaway. FSH builds the follicle, estrogen rebuilds the uterus, LH releases the egg, progesterone prepares for implantation.

Day-by-Day: Ovary and Uterus Changes

Day 14 is the turning point.

Takeaway. Ovulation on day 13–14 splits the cycle into pre-ovulatory (follicular) and post-ovulatory (luteal) halves.

Fertilisation: Where and What

Two gametes fuse — inside the fallopian tube.

Takeaway. Fertilisation happens in the fallopian tube, NOT in the uterus.

Sperm Numbers: Millions Released, One Wins

300 million enter, only one fertilises.

Takeaway. Despite millions of sperms, only one fertilises the egg — the rest are absorbed.

Step-by-Step: From Sperm to Zygote

Acrosome enzymes open the door.

Takeaway. Fertilisation usually occurs within 24 hours of ovulation and restores the diploid (23 pairs) chromosome number.

Implantation: Blastocyst Settles in the Uterus

The embryo digs in — pregnancy begins.

Takeaway. Implantation occurs in the uterus (not the oviduct) and is essential for nourishment and O₂ from the mother.

Zygote to Blastocyst: The Journey

Morula to blastocyst in four days.

Takeaway. The zygote divides as it travels down the fallopian tube, reaching the uterus as a blastocyst by day 4.

Placenta: The Foetus-Mother Connection

Two blood supplies, never mixing.

Foetal **villi** embed into maternal **crypts**, forming the placenta. A membrane always separates foetal and maternal blood — they **never mix**.

Takeaway. Materials exchange across a membrane between foetal and maternal blood — the two bloods never directly mix.

Six Functions of the Placenta

Nutrition, respiration, excretion — all in one organ.

Takeaway. The foetus uses the mother's digestive, respiratory and excretory systems through the placenta.

Don't Confuse Fertilisation Site with Implantation Site

Fertilisation ≠ implantation location.

Takeaway. Fertilisation = fallopian tube; Implantation = uterus. Never swap them.

Reproductive Life: 32 Years, ~416 Eggs

Only 416 eggs ever get a chance.

Takeaway. A female's reproductive window is about 32 years, with roughly 416 eggs maturing in that time.

Exam Focus: Hormones and Phases

Know which hormone does what, and when.

Takeaway. Link each hormone to its phase and target organ — this is the most commonly tested area.