Pregnancy, which is often deemed to be a relatively simple process - the union of egg and sperm, is actually extremely complex involving numerous steps. It starts with the release of follicle stimulating hormones from the pituitary (brain) to the implantation of embryo and to the development of feotus. The process may go wrong at any of the several steps involved causing delay in pregnancy or infertility.
A human cell is composed of 46 chromosomes (diploid); out of which, 22 pairs are autosomes (normal) while the 23rd pzair is a sex chromosome, which determines the sex of an individual. In female there are 44+XX (the sex pair is of similar type) chromosomes and in male there are 44+XY chromosomes. During the process of formation of sperm and egg, the number of chromosomes is halved (haploid). After that, the egg contains (22+X) chromosomes. Out of millions of sperm being continuously produced, half of them contain X chromosome while the other half contains Y chromosome from the XY pair.
Formation and Maturation of Sperm (Spermatogenesis)
In man the process of spermatogenesis (development and maturation of a sperm from stem cell) requires approximately 70 days and passage through the epididymis approximately another 12 days. Therefore, it takes almost three months for a sperm for complete maturation and to be available for ejaculate. The single ejaculate has an average volume of 2-5 mL. The number of sperm in normal semen ranges from 20-100 million/mL, out of which 30% or more are of normal morphology. The World Health Organization (WHO) suggests the following parameters for normal values,
|Volume||2.0 mL or more|
|Sperm Concentration||20 million/mL or more|
|Motility||50% or more with forward progression, or 25% or more with rapid progression within 60 minutes of ejaculation|
|Morphology||30% or more normal forms|
The rate of fertilization strongly depends upon the number, motility as well as the morphology of sperm in a particular semen sample.
As it takes nearly 90 days for the development, maturation and availability of sperm in the ejaculate, the results of any therapy will, therefore, be observable only after 3 months of treatment.Ovulation
The essence of maturation of ovum is reduction of chromosomes to half. The oocyte is covered with layers of cells called primordial follicle. Some of these follicles are selected for ovulation every month to undergo maturation. However only one grows upto the size of 18-25 mm and is finally discharged from the surface of the ovary. This process is called ovulation. The oocyte is picked up by the fallopian tube. In relation to the menstrual period, this event occurs around 14 days prior to the expected period. However, some cycles may be anovulatory (when no egg is formed). It is noteworthy that ovulation may occur randomly each month from either of the ovariesFertilization
Fertilization is a phenomenon involving fusion of sperm with the mature ovum to form zygote. This event occurs in the fallopian tube. Fertilization can occur within 24 hours of ovulation. Sperm can retain the fertility potential in the fallopian tube for 48 hours after ejaculation. Out of millions of sperm deposited in the vagina, finally only 300-500 succeed in making their way to the ovum. It takes about 1 hour for the sperm to reach the site of ovum. Out of a bunch of 300-500 sperm only one succeeds in penetrating the ovum resulting into the formation of a zygote.
Zygote is further divided into the 16-20 celled stage called morula. This process of division takes about 72-120 hours. Morula passes to the uterine cavity where it remains free. In the cavity it further divides to 30-200 celled stage called blastocyst.
It is defined as the process by which an embryo attaches itself to the uterine wall and penetrates the epithelium first and then the circulatory system of the mother to form a placenta. A normal pregnancy is of course impossible without successful implantation and formation of placenta. Implantation of the blastocyst begins 2-3 days after the morula enters the uterine cavity. It burrows more and more inside the uterine wall. The penetration process is completed by 10th or 11th day. The implantation site is usually in the upper wall of uterus.