Menstrual Cycle & The Female Reproductive System | Melbourne IVF

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Menstrual Cycle

Understanding your menstrual cycle

There are typically three phases in your monthly menstrual cycle, controlled by follicle stimulating hormone (FSH) and luteinising hormone (LH). Your cycle could become irregular or cease if the hormonal balance or production is disturbed for any reason.

Phase 1: The follicular phase

Day 1 of your cycle is the first day of bleeding. Your levels of progesterone and oestrogen have fallen towards the end of the previous cycle, so the hypothalamus signals to the pituitary gland to increase production of follicle stimulating hormone (FSH).

Up to 20 to 30 follicles may become active within the ovaries – fewer follicles develop during the cycle as you get older. The developing follicles produce oestrogen, and as the level of this hormone increases FSH production decreases, until there is only enough to encourage further development of one follicle – the dominant follicle.

The increasing amount of oestrogen also causes a build up in the thickness of the lining of the uterus (the endometrium) in preparation for pregnancy, and changes the cervical mucus so sperm can pass through it freely.

Phase 2: Ovulation

As the level of oestrogen increases, there is a surge of luteinising hormone (LH). This hormone triggers ovulation, causing the egg to mature, and the dominant follicle ruptures and releases the egg.

As the egg is released, the fimbriated end of the fallopian tube moves across the ovary and collects the egg. If fertilisation (or conception) occurs, it is normally in the widest part of the tube, near the ovary, called the ampulla.

For a few days before ovulation, the cervical mucus allows sperm to pass through the cervix and uterus to the fallopian tubes. It can survive there for two to three days.

A shell called the zonapellucida surrounds the egg and hardens after one sperm has penetrated it, so other sperm cannot enter. Once inside the egg the sperm releases its contents and fertilisation occurs. The fertilised egg starts to divide into cells, the number of cells doubling with each division, and becomes known as an embryo.

Phase 3: The luteal phase

The follicle from which the egg was released now begins to make progesterone. This hormone enables the endometrium to provide nutrition, and a site for the embryo to implant during pregnancy.

The embryo moves along the fallopian tube and once it reaches the uterus it hatches out of its zonapellucida shell about five to six days after fertilisation, and implants in the endometrium.

The embryonic cells that will form the placenta produce human chorionic gonadotrophin (hCG) – the hormone measured in pregnancy tests. The presence of this hormone drives the corpus luteum (as the follicle is now known) to continue making oestrogen and progesterone to support the pregnancy, until the placenta can take over.

If fertilisation fails or does not take place, in the absence of hCG the corpus luteum will shrink, causing oestrogen and progesterone levels to fall. Without these hormones, the endometrium breaks down, menstruation starts and the menstrual cycle starts once more.

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