The start of the
menstrual cycle is defined as the first day of bleeding. Every month, one of the two
ovaries produces one mature
ovum enclosed within a
follicle. During the previous cycle already, several follicles (on average 10 -12) start preparing for next month’s
ovulation. Most of the time, only one egg matures within a follicle while the other follicles degenerate and are subsequently re-absorbed.
A mature follicle is about 2 cm and is filled with follicular fluid. The ovum is located in the follicle’s septum. The follicle reaches its maximum size by mid-cycle. By day 14 the follicle ruptures and together with follicular fluid an ovum is released into one of the
fallopian tubes – this is referred to as ovulation. Aided by very fine muscle contractions in the Fallopian tube and constantly beating microscopic hairs (celia) in the inner lining of each tube, the egg is transported towards the
uterus.
1. The normal menstrual cycle
Having unprotected sexual intercourse around this time, the
sperm cells released into the vagina during ejaculation travel through the uterus into the ampullary part of the fallopian tube, where the ovum is fertilized by one single sperm.
The fertilized ovum, now referred to as the
embryo, already starts dividing (cleavage) on its way through the fallopian tube towards the uterus. On average by day 5 after ovulation the embryo reaches the cavity of the uterus, where it starts implanting into the uterine lining.
Each menstrual cycle anew the uterine lining (endometrium) becomes thicker and generates a richer supply of blood vessels, thus preparing for the implantation of the embryo. The developing follicle produces the hormone “
estrogen”. The larger the follicle, the higher the estrogen level rises. This hormone is responsible for thickening the uterine lining before ovulation.
After ovulation has occurred the remnants of the ruptured ovarian follicle convert into what is commonly known as the yellow body (corpus luteum) (after ovulation the yellow body develops from the granulosa cells which formed the inner lining of the cavity of the ovarian follicle). The corpus luteum secretes the hormone “
progesterone”, which is preparing the uterine lining so that by the time the embryo reaches the uterus, the lining is ready for the embryo to implant.
The embryo then starts secreting the hormone
HCG (HCG is the pregnancy hormone, giving the positive result on both urine and blood based pregnancy tests).
Within the first 10-12 days after the implantation, the embryo is still so small, that HCG isn’t detectable. For the yellow body, indeed, the signals are strong enough to induce its growth, and with it, the production of more and more progesterone. Thus, the yellow body has a key role in maintaining the thickness of the uterine lining. This process is of crucial importance, because low progesterone levels can cause bleeding.
Corpus luteum insufficiency
Due to the absence of an implanted embryo and its missing signals, it is quite normal that progesterone levels start declining. The yellow body withers and disintegrate, the uterus sheds its lining and a new menstrual cycle begins. The onset of the period can be regarded as the start signal for a new menstrual cycle. This is controlled by a complex regulatory mechanism that involves
pituitary gland (hypophysis), ovaries and particular areas of the brain.