A woman is born with all the eggs she will ever have – they remain immature within very small follicles in the ovary. At the beginning of your period, the pituitary gland in the base of your brain releases FSH (follicle stimulating hormone) and LH (luteinising hormone).

A number of small immature follicles start to grow (this number depends largely on your ovarian reserve), but in most cases only one follicle will grow to be ‘dominant’ and to contain a mature egg (oocyte). The rest of the follicles that had been recruited initially will die away, and these eggs are not useful for reproduction. As the dominant follicle grows, it produces oestrogen, it is this hormone that leads to growth of the endometrium (lining of the uterus). When the follicle reaches a size of maturity – about 18-20mm in diameter, and when the oestrogen level is adequate, the pituitary gland releases a surge in LH (and to a lesser degree FSH). It is this surge in hormone that kick starts the process of ovulation. Ovulation is the release of the egg from the follicle – it occurs, on average, about 40 hours after the start of the surge.

After release of the egg, the cells that line the follicle then start to produce progesterone. The follicle is then called a corpus luteum. It is the progesterone that ‘stabilises’ the endometrium and starts changes that make the endometrium suitable for implantation if a fertilised egg reaches the cavity. If a pregnancy results, the pregnancy hormone continues to stimulate the corpus luteum to produce progesterone, and it is this that allows the endometrium to remain stable and for the pregnancy to implant. If you are not pregnant, there will be no ongoing stimulation of the corpus luteum, the progesterone level drops, and eventually menstruation will occur. This is the luteal phase, and lasts on average 12-14 days. It is the luteal phase that is most consistent and predictable.

The process then begins again.


Normal Menstrual Function

The menstrual cycle is divided into two phases – the follicular phase up to ovulation, and the luteal phase following the release of the mature egg.

Diagram from the American Society of Reproductive Medicine:
Diagram from the American Society of Reproductive Medicine


Oocytes

An Oocyte (egg) needs to be exposed to the LH surge, or it will not be able to go through the last stage of maturity. Once a mature egg is released by the ovary, it is ‘picked up’ by the fallopian tube, and moved towards the endometrial cavity. It is along this path that it will hopefully meet the sperm. The egg will reliably survive in this environment for 12 hours, and may live up to 24 hours. As sperm usually survives for 2 days (and up to 5 days) Therefore, if a couple is having intercourse every second day around the day of ovulation (even if you are not completely certain of timing), then there will be a good chance that egg and sperm will meet.