Roles of FSH & LH in the Menstrual Cycle

Phase 1: Follicular Development (Days 1-13)

• FSH Dominance: During the start of the cycle, FSH levels are elevated to 'recruit' and mature a primary follicle. This is the preparation phase where the genetic material is readied for release.
• Oestrogen Induction: As the follicle grows under FSH stimulation, it releases increasing amounts of oestrogen, which thickens the endometrium and prepares the body for LH release.

Phase 2: Ovulation (Day 14)

• The LH Surge: Triggered by peak oestrogen, a sharp spike in LH occurs over a 24-48 hour window. This surge causes the follicle to rupture, releasing the egg into the oviduct where fertilisation can occur.

Phase 3: Luteal Phase (Days 15-28)

• Progesterone Production: LH stimulates the empty follicle to become the corpus luteum, which secretes progesterone. If fertilisation does not occur, the corpus luteum degenerates, leading to a drop in hormones and the start of menstruation.

• It is essential to distinguish between the 'preparatory' role of FSH and the 'action' role of LH to understand why they peak at different times.

• Inhibition vs. Stimulation: Note that oestrogen has a dual effect on pituitary hormones. It inhibits FSH (to prevent multiple ovulations) but stimulates the LH surge once a certain threshold is reached.

• Identify the Source: Always clarify that FSH and LH come from the pituitary gland, not the ovaries. Examiners frequently use this as a distractor in multiple-choice questions.

• The 'Surge' Keyword: When describing ovulation, use the term 'LH surge'. Ovulation is not a slow process; it is a rapid event triggered by a dramatic increase in LH concentration around day 14.

• Link to Feedback: Be prepared to explain how oestrogen levels affect FSH and LH. Remember the sequence: $FSH \rightarrow Follicle \rightarrow Oestrogen \rightarrow LH \rightarrow Ovulation$.

• Graphical Analysis: Practice reading hormone level graphs. FSH usually has a small bump at the start, whereas LH will have a very tall, sharp peak exactly at the midpoint of the 28-day cycle.

• FSH vs. LH Roles: A common mistake is thinking FSH causes ovulation. FSH only prepares the egg; without the LH surge, the mature egg will stay trapped inside the follicle.

• Source Confusion: Many students incorrectly state that FSH is an ovarian hormone because it affects the ovaries. Always remember that it is a messenger from the brain to the ovaries.

• Pregnancy Maintenance: Don't forget that if pregnancy occurs, the corpus luteum (stimulated by LH) must continue producing progesterone until the placenta is ready to take over.