First half of the cycle (days 1 to 14)
When FSH is increased, one to three eggs (ova) start growing to maturity. Each of these eggs is located in a follicle, which is responsible for production of the female sex hormone (oestrogen). This hormone causes the uterine lining (endometrium) to be formed and the cervix (the neck of the womb) to open slightly. It also promotes the production of cervical secretion, which facilitates penetration of the sperm into the uterine cavity.
Middle of the cycle (approx. days 14 to 16)
During mid-cycle, the follicles are fully matured and produce maximum amounts of oestrogen. This increase in oestrogen causes an increased secretion of LH by the pituitary gland. The sex hormone ensures that the follicle breaks and the egg is released from the ovary – this is ovulation. Yellow bodies, which produce the hormone progesterone, are formed from the remains of the follicle. In the event of fertilisation, progesterone helps to sustain pregnancy. The mature eggs are caught by the fallopian tubes and transported in the direction of the uterus. They remain able to survive and capable of being fertilised for 24 hours.
Second half of the cycle (approx. days 17 to 28)
The egg can be fertilised while it is in the fallopian tube. If a sperm reaches the egg and succeeds in penetrating the embryonic membrane, the egg divides several times before it has even reached the uterus. After another four to five days, the embryo is implanted in the lining of the uterus. The uterus immediately starts sending out the messenger hCG (human chorionic gonadotrophin, the so-called pregnancy hormone) to sustain the pregnancy.
If the egg is not fertilised in the fallopian tube, the ovary recognizes this, since the hCG signals fail to be sent out from the uterus. The yellow body, which is formed from the follicle remains, perishes in 10-14 days. This causes the progesterone levels in the woman’s blood to drop and the lining of the uterus bleeds out. That signals the start of the first day of the new cycle.