During the natural fertilization process, follicles containing eggs develop in the ovary. Once a month, an egg is released from the ovary into the fallopian tube where it can meet with a sperm cell. After fertilization in the fallopian tube, the fertilized egg develops into an embryo and is then transferred to the uterus within 4-6 days and implants in the endometrium.
The female reproductive system consists of the external genitals and the internal genitals. The external organs include the labia majora, labia minora, the clitoris, urethra and hymen located near the entrance to the vaginal canal. The internal organs include:
- Vagina: a flexible channel surrounded by muscles. It is 10 cm long and it is through the vagina that the penis penetrates during sexual intercourse. Additionally, the vagina also helps the body drain menstrual blood from the uterus.
- Cervix: the cervical canal is the passage between the vagina to the uterus. The cervix allows the timely penetration of sperm to the uterus for ovulation and also protects against penetration of infections.
- Uterus: the uterus is a hollow, pear-shaped organ with thick muscular walls. The normal length of the uterus is about 8cm. The uterine cavity is lined with mucosa that allows the implantation of the embryo. This endometrium develops each month during menstruation and until ovulation.
- The endometrium, the innermost layer of the uterine walls, thickens due to estrogen. Afterwards, due to progesterone, secreted after ovulation, the mucosa undergoes a change preparing it for pregnancy. If pregnancy does not occur there is degeneration and peeling off of the mucosa accompanied by menstrual bleeding, called the 'period'. Defects in the development of the endometrium may impair the chances of embryo implantation.
- The Fallopian Tubes: The fallopian tubes are a pair of delicate tubules with a length of about 10 cm, branching from the upper corners of the uterus and reaching up to lie next to the ovaries. The fallopian tubes play an important role in the transfer of the egg and sperm, and fertilization occurs within them. At the edge of the wide funnel of the tubes are structures resembling petals (fimbriae), fixed near the ovaries and these absorb the ovulating egg, after which the egg has no self-mobility and depends on the function of the fallopian tube for its transfer from the ovary to the the uterus.
- Ovaries: The ovaries are a woman's egg repository and contain about 300,000-400,000 eggs. Within them, the eggs mature until ovulation and the female sex hormones are secreted from them. Unlike the constant creation of sperm cells in the testes, the number of eggs available in the ovaries does not renew and is constantly decreasing.
The woman's reproductive system supports the process of egg maturation and ovulation, fertilization and embryo implantation. This complex operation is made possible thanks to a large number of hormones, proteins and growth factors.
Every month several follicles mature in the ovary, each containing fluid rich in hormones and a single egg. Of all the follicles, one will reach full maturity, and from this the corresponding egg will be released for fertilization the same month. The process, in which the mature egg is released from the ovary, is called ovulation.
The ovulation begins in the pituitary gland in the brain, which secretes two hormones into the bloodstream and from there to the ovary: FSH - follicle stimulating hormone, stimulating the development of the follicles in the ovaries, and LH - luteinizing hormone, which causes the final maturing of the egg and stimulates the onset of ovulation. To induce ovulation in the framework of fertility treatments, the hormones can be injected in a preparation extracted from the urine of menopausal women or from those produced through genetic engineering methods.
The ovarian follicle develops and grows at a rate of 1-2 mm per day up to 15-30 mm until its expiration and release of the egg. During this process, the hormones estradiol and progesterone are secreted by the follicle. Therefore, in addition to measurement using ultrasound, it is possible to monitor follicle development by taking measurements of the levels of estradiol (E2) in the blood. These hormones are responsible for, among other things, preparing the endometrium for the absorption of the embryo.
Fertilization of the Egg
After the egg is released from the follicle, it is collected by the fallopian tube and waits for the arrival of the sperm cells. The lifespan of the egg is 24-48 hours. If it is not fertilized, the egg will degenerate and disintegrate. If the egg meets with the sperm cells, many spermatozoa will try to penetrate inside it. But once one sperm has entered, a reaction is immediately created in the egg membrane that prevents further penetration of sperm.
After fertilization, the fertilized egg (zygote) begins dividing and the resulting embryo continues its journey toward the uterus. The embryo reaches the uterus after 4-6 days and will number over a hundred cells. These cells are arranged in the shape of a strawberry with a central cavity (blastocyst), some of which will develop into an embryo and the rest into the future placenta membranes of the pregnancy sac.
Implantation of the Embryo
For the fetus to continue to develop, its budding from the surrounding envelope and its implantation in the endometrium is now required. After implantation, a connection is made with maternal blood vessels to nourish the embryo and further its development. About three weeks after ovulation a tiny embryonic sac can be detected in utero by ultrasound examination and after another week a fetal heartbeat can be detected, a sure sign of the vitality of the fetus.