Menstrual Cycle
Did you know the menstrual cycle is more than a PERIOD?!
Let’s start with a BASIC explanation of The Menstrual Cycle:
Typically a menstrual cycle lasts between 24 and 38 days. A cycle is calculated from the first day of menstruation (period) up to the first day of the next period. It is made up of two distinct phases: the follicular phase and the luteal phase.
Follicular Phase:
The period or first day of menstruation marks the beginning of the follicular phase. During this phase, the uterine lining thickens as it prepares for receiving a fertilised egg.
Ovulation:
Ovulation occurs at the end of the follicular phase, where the follicle (that has matured in the ovary) releases an egg. This is typically half way in your cycle, and distinctly divides the two phases.
Luteal Phase:
The luteal phase begins after ovulation so the egg can be fertilised. If the egg is not fertilised, the uterine lining sheds with a drop in oestrogen and progesterone - and your period beings (starting the cycle again).
Okay, let’s explore The Menstrual Cycle in more DEPTH:
Follicular Phase:
The development of ovarian follicles defines this phase which begins from the first day of menstruation until ovulation.
The pituitary gland at the base of the brain releases follicle stimulating hormones (FSH) causing stimulation of ovarian follicles. Follicles are sacs that develop of the surface of the ovary, each containing an oocyte (or egg). As the follicles mature, one will become dominant and fully mature, this begins to produce large amounts of oestrogen.
The oestrogen hormone plays a large role here - it inhibits the growth of other competing follicles, results in endometrial thickening and thinning of the cervical mucous (allowing easier sperm movement).
Oestrogen will exceed a threshold level to stimulate the production of luteinising hormone (LH) from the pituitary gland.
Ovulation:
The high levels of LH will thin the membrane of the mature follicle. Approximately 24-48hours after the surge in LH, the follicle will rupture releasing an oocyte. This oocyte will quickly mature to become a mature ovum that is released and taken into the fallopian tube by fimbriae (little finger projections).
Luteal Phase:
Following ovulation, the opened follicle will seal over and develop into the corpus luteum. The corpus luteum is responsible for producing the hormone progesterone.
Increased levels of progesterone result in the endometrium (lining of the uterus) becoming receptive to implantation of the blastocyst and increase the basal body temperature.
If fertilisation doesn’t occur - Menstruation:
Progesterone will cause a negative feedback loop to decrease the LH and FSH, causing the corpus luteum to disintegrate. With the disintegration, progesterone levels will drop causing arteries in the endometrium to contract. This will result in triggering the endometrium to shed, and be discarded from the body as menstruation.
If fertilisation does occur - Pregnancy:
As ovum is fertilised by the sperm, it will produce human chorionic gonadotropin (hCG) - which is similar to LH in function. The hCG will prevent the corpus luteum from disintegrate (meaning a continued production of progesterone). As progesterone is continually produced, it will prevent menstruation. The placenta will take over the role of the corpus luteum (from ~8weeks gestation).