The first step of pregnancy usually begins with sexual intercourse where male gametes or sperm are deposited into the vagina. The semen produced by the male contains not only sperm cells but also sugars, proteins and other substances to help keep the sperm viable. Human sperm generally survive for about 48 hours in the female body. Sperm have a long flagellum, which they use to swim; they are the only human cells with this property. These cells are haploid, having divided by meiosis from germ cells in the testes and possessing only one half of the chromosomes of ordinary body cells. Typically, between 100 million and 300 million sperm are released in one ejaculation.
Ova, or oocytes, are the haploid female egg cells, and their role is to fuse with one sperm cell to form a fertilized zygote which will then grow in the uterus to form a developing fetus. These cells are produced by meiosis in the ovaries and stay in a state of suspended animation until activated by hormonal changes in the woman’s menstrual cycle. Typically, only one ovum is released during each menstrual cycle.
At ovulation, the fimbriae at the end of the fallopian tube move over the ovary to catch the released ovum. If fertilization takes place, the sperm usually meet the ovum in the fallopian tube, requiring the sperm cells to swim from the upper vagina through the cervix and across the length of the uterus before reaching the fallopian tube—a considerable distance compared to the size of the sperm cell.
Once there, the successful sperm swim towards the ovum and each attempts to fertilize it. Each sperm cell contains a small pouch of enzymes that it uses to break through the outer layer of the ovum in order to fuse with it. This can take up to twenty minutes. Once the ovum fuses with a single sperm cell, its cell membrane changes, preventing fusion with other sperm. The fusion of the ovum’s and sperm’s nuclei to form a diploid cell completes the first stage of pregnancy.
Alternative methods of fertilization, including artificial insemination and in vitro fertilization, are sometimes used in cases of infertility and by single women and lesbian couples.