Conception occurs after sexual intercourse between male and female during ovulation (the female fertile period) when the egg (the female sex unit) and the spermatozoa (the male sex unit) are joined and fused.
In the middle of the woman’s menstrual cycle, about 2 weeks after the previous or 2 weeks before her next period, a mature egg is released from her ovary and travels through the Fallopian tube to the uterus for 2 to 3 days, then the egg is ready for fertilization. This is the time of ovulation (the female fertile period) and it lasts between 12 hours and up to several days.
If a woman has sexual intercourse during she’s fertile period and neither she nor her partner has taken contraceptive measures to prevent the fertilization, then several million spermatozoa reach and enter the uterus through its opening – the cervix. They move in the direction of the released egg in order to reach and fertilize it.
Many spermatozoa die on their way because of the vagina’s acidic environment, but only one is enough to fertilize the egg. The sperm that enters the uterus can survive for 5 days maximum before being disintegrated from the woman’s immune system.
More sperm cannot fertilize an egg because, after the penetration of the first one, the fertilized egg instantly produces an impenetrable membrane (zona pellucida) and prevents others from entering.
After the egg is fertilized by the sperm, their cores fuse into one and their substances are merged, commencing the pregnancy of the woman.
The spermatozoa and the egg are carrying chromosomes that contain the genes (carriers of the characteristics of the woman and the man). In the newly formed nucleus (the conception of the new life), each hereditary feature is represented with two genes, one of the male and another of the female. Sometimes one will prevail, another time the other, but each inherited trait is the result of their combination.