Menstrual cycle

What happens during the menstrual cycle in our body? How do uterus, ovaries and brain cooperate to make this incredible process happen?
The menstrual cycle is counted from the beginning of menstruation. It means that 1st day of menstruation equals 1st day of cycle.

1st day:
From our birth each ovary contains a set of ovarian follicles with all our immature eggs. Every month the brain activates about 20 follicles, which grow and mature.
The first day of the cycle is quiet for ovaries, which produce very little hormones. The brain (the pituitary gland to be exact) produces the hormone FSH. It is a sign for the ovaries to choose 20 ovarian follicles and start feeding them 
Around the 10th day one of the 20 ovarian follicles is chosen
(sometimes more – hence multiple pregnancies), the rest wither and die - it has not been determined how this choice is made.
The ovarian follicle fills with a hormone - estradiol, which helps reconstruct the uterus lining after menstruation. At the same time when the egg is growing so is the uterus lining, preparing for the embryo.
The egg is getting bigger, in the last stage the ovarian follicle is 2.5 cm wide and 1.5 cm high! (Bearing in mind that the ovary is no bigger than an almond it is impressive!)
In the meantime “the fallopian tubes, those gorgeous pink sea pens, follow the drama with their feather-duster tips”. To determine the phase of the maturing egg they “brush over the surface of the ovaries”. 

12-14th day – the pituitary gland gives a signal for ovulation (freeing the eeg) by producing hormone LH, which makes the ovarian follicle rupture – it can cause slight bleeding - so called ovulation spotting and mild cramps.

The egg is freed from the ovary. The endings – fimbriae - of fallopian tubes contract rythmically and pull the egg inside, where it starts travelling deeper.
The ruptured ovarian follicle lives on – like a mother after giving birth, it wants to feed its ‘baby’ so it produces hormones: “the cells lining the pit swell, fill with cholesterol, and turn soft and yellow, like butter or custard”. This way corpus luteum (yellow body) is created. It produces a lot of progesteron and a bit less estrogen, which stimulate the uterus lining to thicken and make breasts a little swollen and sensitive.
If the egg encounters sperm and is fertilized, corpus luteum produces hormones (mainly progesteron) for another 42 days to keep the embryo alive until the placenta grows and takes over this function. But even later it doesn’t disappear – in the words of Natalie Angier “it is still the dominant follicle, the crowned queen, who keeps other follicles on the ovaries chastened, immature”. In this way, once we are pregnant we cannot get pregnant with another baby  Corpus luteum feeds the mother as well: it spurts fat yellow tissue and feeds bones, kidneys, pancreas and the brain.

An egg outside an ovary can live for 24 hours. When can I get pregnant? Which days in the cycle are fertile?
An egg lives up to 24 hours. If it doesn’t get fertilized by sperm in that time, it dies. So, we can get pregnant during the 24 hours of our mature egg’s life.
However, since sperm can live in the uterus from 3 to 5 days (some sources claim that it can be up to 7) the 5 days before freeing an egg (ovulation) are called fertile time.
How to recognize fertile days?

22-24th day (10 days after ovulation) – if the egg wasn’t fertilized, the corpus luteum decays, the follicle attracts macrophages – immune system cells which remove dying or dead cells from the body. The ovarian follicle grows over with fibrous scar tissue and corpus albicans (white body) is created.

28-29th day - Menstruation begins – the inside layer of uterus – the endometris, which grew to keep and feed the potential embryo, dies and is shed - like a snake’s skin. It flows out of the body with blood, making space for a new life, and so the cycle starts anew...

What exactly happens during menstruation?




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