Junior Science | Human Reproduction and Puberty: Tutor Prep Guide (28-Day Cycle Maths, Four Errors, Exit Check)

Set the principle first: reproductive organs do two jobs only, making gametes and bringing them together. In males the testes make sperm and secrete testosterone, which is why they sit outside the body where it is two to three degrees cooler; the sperm duct transports and the prostate adds fluid. In females the ovaries make eggs and secrete oestrogen and progesterone, the oviduct is where gametes meet, and the uterus is where implantation and pregnancy occur.

Numbers for contrast: one ejaculation carries roughly 200 to 500 million sperm, each about 50 micrometres long; a woman releases about 480 eggs in a lifetime, twelve a year for forty years, each about 100 micrometres across and unable to swim. Summary: sperm many and small and motile; eggs few and large and carried.

Students repeat: sperm are made in the testes, eggs in the ovaries, and each carries only 23 chromosomes.

Draw a 28-box axis. Days 1 to 5 are menstruation, when the lining breaks down. Days 6 to 13 are the follicular phase, when an egg matures and the lining thickens. Day 14 is ovulation. Days 15 to 28 are the luteal phase, when the lining waits for a zygote and then breaks down if none arrives.

There is one formula: ovulation day equals cycle length minus 14. A 28-day cycle gives day 14, a 30-day cycle gives day 16, a 35-day cycle gives day 21. The luteal phase is fixed at about 14 days, so the variable part is the first half, which is why we count backwards rather than forwards.

Then why rhythm methods fail: sperm survive three to five days in the female tract, an egg about one day, and cycle length shifts with stress, illness and sleep. Avoiding day 14 is therefore not contraception. Say this plainly and without judgement.

Fertilisation happens in the oviduct, not the uterus, and that is the single most common error. Sperm swim through the uterus into the oviduct where they meet the egg; the two sets of 23 chromosomes combine to make 46. Cleavage follows, and implantation in the uterine lining occurs around day six or seven.

Forty weeks, 280 days, divides into three trimesters of about 13 weeks each: the first, when organs form and sensitivity is highest; the second, when bone and muscle develop and movement is felt; the third, when weight rises quickly and the lungs mature. The placenta and umbilical cord exchange oxygen, nutrients and waste, and maternal and fetal blood never mix directly.

Close with three sentences the class repeats together: half the chromosomes from each parent, fertilisation in the oviduct, forty weeks of gestation.