Menstrual cycle
Ovulation is the result of a series of events that results in the production and the discharge of an ovum from an ovarian follicle.
The pregnancy begins when the ovum is fertilized and adheres to the thickened wall of uterus (endometrium).
The process of fertilization can only occur during a specific period of time which is called “fertile phase of the menstrual cycle”.
If fertilization does not occur, the endometrium will shed as menstrual blood in the beginning of the next cycle.
To achieve a pregnancy
1) The balance of the hormones must be at the proper levels so that the ovum will develop normally.
2) An ovulation must occur
3) Sexual intercourse must occur during or very close to the ovulation day (the so called “fertile day of the cycle”)
4) Semen must have a sufficient number of motile spermatozoa in order to fertilize the ovum.
5) There must not be an obstruction in the oviduct (fallopian tube) preventing the insemination and the implantation.
These events are controlled by an endocrine gland, the pituitary gland.
The cycle can be divided into 2 phases:
Follicular Phase.
In the beginning of the cycle Follicular Stimulating Hormone (FSH) is released from the pituitary gland. FSH induces egg maturation.
During the next 12-14 days the oocyte develops and matures.The ovary produces an increasing amount of estrogen.
As a consequence...
1) Estrogens are recognized by special receptors in the brain. If the production of estrogens is sufficient the secretion of FSH is self-controlled and the levels of FSH are decreased. This allows the development of a single ovum (oocyte).
2) Later, the estrogen levels induces the secretion of Luteinizing Hormone (LH) from the pituitary gland. This hormone is responsible for the release of the ovum (ovulation).
Luteal Phase
After ovulation is called the luteal phase. Increased levels of estrogens -and another female hormone progesterone -act in the endometrium and make the lining thicker (in preparation for implantation).
Following the release of the oocyte from the ovarian follicle a "corpus luteum" develops from the empty follicle and produces progesterone.
If the fertilization doesn’t occur the levels of estrogen and progesterone decreases, the endometrial lining will be shed and a new cycle begins.
If implantation has occurred, the corpus luteum continues to produce progesterone to aid the developing pregnancy.
The drugs used in ART are imitating the woman’s natural cycle-
-FSH is given by injection.
-A similar hormone to LH, called HCG, is given at the correct time in the IVF cycle to mature the oocytes.
-Progesterone is given in order to help implantation and to maintain the endometrium.
