Reproduction in Humans
Senior 3 · Biology
Reproduction in Humans
Lesson 3 of 5
25 min
Every month, from puberty to her late forties or early fifties, a woman's body prepares for a possible pregnancy. An egg ripens and is released, and the lining of the uterus thickens to receive an embryo. If no embryo arrives, the lining is shed and the whole cycle starts again. The bleeding that results, a period, is a normal and healthy part of life, not something shameful. In Senior 1 you learnt how to manage periods hygienically; this lesson explains the four hormones that time the cycle.
Definition
Menstrual cycle: the repeating series of changes in the ovaries and uterus, typically about 28 days long (commonly between about 21 and 35 days in adults, and often irregular in the first years after periods begin).
Menstruation (a period): the shedding of the uterus lining, with some blood, through the vagina, typically for about 3 to 7 days. Day 1 of the cycle is the first day of bleeding.
Ovulation: the release of an egg from an ovary, typically around day 14 of a 28-day cycle.
Follicle: a small fluid-filled sac in the ovary in which an egg matures. After ovulation the empty follicle becomes the corpus luteum ("yellow body"), which makes progesterone.
Figure 1: typical curves for a 28-day cycle; heights are not to scale, and real cycles vary in length and timing.
| Hormone | Made by | Main effects in the cycle |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Makes a follicle and its egg mature in the ovary; stimulates the ovary to make oestrogen |
| Oestrogen | Ovary (the growing follicle) | Makes the uterus lining thicken again after menstruation; at high levels, triggers the LH surge; inhibits FSH |
| LH (luteinising hormone) | Pituitary gland | Its sudden surge causes ovulation; makes the empty follicle become the corpus luteum |
| Progesterone | Corpus luteum (ovary) | Keeps the lining thick and ready for an embryo; inhibits FSH and LH, so no new egg matures |
This is an example of hormones controlling each other by feedback: progesterone and oestrogen, made because of FSH and LH, in turn switch off FSH and LH.
Worked example 1: reading Figure 1
Worked example 2: cycles of different lengths
Ovulation typically happens about 14 days before the next period starts, whatever the length of the cycle. Esther's cycles are 32 days long. On about which day of her cycle does she ovulate, and how many periods does she have in a year?
Warning
Common mistakes. (1) Saying FSH and LH are made by the ovaries: they are made by the pituitary gland. (2) Writing that menstruation happens at ovulation: they are about two weeks apart. (3) Thinking every cycle is exactly 28 days: lengths vary, especially in teenagers.
Health. Very heavy bleeding, periods that stop for several months (when not pregnant), or severe pain that stops normal activities are reasons to see a health worker. No girl should be kept out of school because of her period; ask a teacher or the school nurse for help with pads if you need them.
Menstrual Cycle Hormones, Launchpad Learning. Watch for the roles of FSH, oestrogen, LH and progesterone, and check them against the table and Figure 1.
Tip
Memory hook: "F-O-L-P" in order of their peaks. FSH (early), Oestrogen (before day 14), LH (day 14), Progesterone (around day 21).
Practice: the menstrual cycle
Key points
Sources and credits
Written by Ganzaa: the text, Figure 1 (typical curves), the table, the worked examples and the practice questions.
