Natural insemination occurs during sexual intercourse. The lower part of the uterus, which leads to the vagina, is called ectocervix. Its patency for sperm is influenced by the mucus which changes appearance and quantity during the menstrual cycle due to the hormonal influences. At midcycle around the time of ovulation the mucus is copious, clear, viscous and enables sperms an easy path to the uterine cavity.
From the uterus sperms must reach the fallopian tube where the ovum is inseminated. Most of the sperms die on the way; nevertheless, many of them encounter the ovum. In the female reproductive system the sperms stay active for 5 days although they can "sleep" for a certain time until they are reactivated by the released egg. Ideally, a single sperm overcomes the barrier of the zona pellucida (egg coat) and inseminates the egg. Biochemical and structural changes occur in the ovum and they prevent other sperms from penetrating.
Life before life
After the sperm has penetrated into the ovum, a zygote is formed. It consists of two pronuclei - one from the father and the second from the mother. Subsequently, both pronuclei fuse and the embryo starts to split. Proceeding through the fallopian tube results in the cleavage of the embryo which comes to the uterus at the stage of the blastocyst and approximately on the sixth day it embeds itself into the endometrium.