From Encyclopedia of Sex and Sexuality
Conception begins when a woman’s egg and a man’s sperm meet within her body to form the first cells of a new being like its parents. If successful, the meeting of egg and sperm in one of the woman’s fallopian tubes results in fertilization of the egg and growth of the first cell clusters of the new being’s embryo. The fertilized egg then moves through the fallopian tube and implants itself in the woman’s uterus (womb). When this process is complete, conception is said to have taken place.
The woman’s egg is present in the fallopian tube and is fertile for approximately three days after she has ovulated (released the egg from an ovary), usually at about the midpoint of her monthly cycle. The man’s sperm, which can remain alive and mobile for up to seven days, is present in the tube because the couple have experienced intercourse during that time. (Although desired, conception often fails to occur for a variety of reasons. However, it can be aided by modern medical techniques, among them artificial insemination and in vitro fertilization, in which important steps of the process occur outside the woman’s body.)
When the egg is ovulated (released from a follicle or fluid-filled sac in the ovary), it is surrounded by many cells produced by the ovarian follicle. Within those cells and immediately surrounding the egg is a tough membrane, termed the zona pellucida, which represents the major barrier to penetration by the sperm. Fimbria (fringes) at the end of the fallopian tube aid in the transfer of the egg from the surface of the ovary and transportation of the egg to the upper region of the fallopian tube. Simultaneously, sperm have been transported from the vagina to the same region of the fallopian tube.
With normal sexual intercourse, millions of sperm are deposited in the upper vagina. Typically, only a few hundred actually reach the site of normal fertilization. Using a combination of swimming motions and release of chemicals, some sperm are able to penetrate the cells surrounding the egg, but usually only a single sperm is able to penetrate the zona pellucida and actually fertilize it. The fertilization process involves fusion of the sperm with the egg itself.
Within the first twenty-four hours after fertilization, the genetic material—chromosomes—carried by the sperm merge with the chromosomes carried by the egg and, approximately thirty hours after fertilization, the fertilized egg divides for the first time. It is now called an embryo. Subsequent cell divisions occur at about twelve hour intervals. Under normal circumstances, fertilization occurs in the outer half of the fallopian tube and the embryo spends about three days floating free in this portion of the fallopian tube. The embryo, now consisting of approximately twelve to sixteen cells, is then rapidly transported into the uterine cavity. It is nourished by uterine secretions and undergoes additional development. Approximately six days after ovulation and fertilization, it “hatches” (emerges from the zona pellucida), expands dramatically, and finally burrows into the endometrium—the wall of the uterus. The new embryo produces a hormone (hCG) which, when released into the mother’s bloodstream, signals that she is pregnant and causes the corpus luteum in the ovary to continue producing the estrogen and progesterone hormones necessary for maintenance of the pregnancy.
The actual process of conception has to be accurately timed. The egg has a very limited life-span in which it can be fertilized (estimated to be no greater than three days). Sperm, on the other hand, are able to maintain their fertilization ability for a much longer period of time. Actually, sperm are probably able to maintain their motility (ability to swim) as well as fertilizing ability in normal cervical mucus for several days. Thus, even though a couple does not have sexual intercourse exactly at the time of ovulation, pregnancy may occur if the woman has had intercourse any time in the several days before ovulation.