Then things start to get a bit confused. It all depends on whether the secondary oocyte is fertilised or not.

Fertilisation

After fertilisation, the secondary oocyte undergoes its second meiotic division to produce the ovum. The zygote is formed and cleaves accordingly. Eventually, the morula and blastocyst are formed

Implantation would then occur at days 20-28 of the menstrual cycle, the 6th day of the secretory phase. Human chorionic gonadotrophin (hCG) is then secreted by the syncythiotrophoblast to maintain the corpus luteum, which continues to produce progesterone and oestrogen.

As a result, the secretory phase continues and menstruration doesn't occur.

No Fertilisation (or the torture that women go through every month... until the menopause)

With no hCG to maintain the corpus luteum, it then degenerates. Concequently, oestrogen and progesterone concentrations go thourgh the floor. There is a bit of confusion about what happens next, some physicians think that there is an ischaemic phase, where the endometrium necroses and falls away. Other physicians consider this as a load of defecatory material.

Either way, mentruation occurs.

Menstruation's effects on the endometrium

Glandular secretion stops, leading to a crash in the concetration of oestrogen and progesterone. There is then a loss of interstital fluid, and as a concequence, the endometrium shrinks.

The spiral arteries constricts, leading to:

• Venous stasis
• Patchy necrosis
• Vessel walls rupturs
• Blood seeps into the connective tissue

Pools of blood then form which break through the endometrial surface, leading to bleedin into the uterus and vagina. This is menstrual flow.

Details of menstrual flow

• ~20-80 mls of blood
• Lasts for 3-5 days
• The compact and spongy layers are shed, and the basal layer remains to aid in the rebuilding of the shed endometrium

In pregnancy, the menstral cycle ceases, and the endometrium enters the pregnancy phase.