Conception takes place when a sperm fertilizes an ovum in the outer one-third of the fallopian tube. This outer one-third is called the ampullar portion. Normally, one ovum matures each month (at the time of ovulation). The ovum needs to be fertilized within the first 24 hours (48 hours at the outermost); if not the ovum disintegrates. The life of a sperm is usually 2 to 3 days; therefore successful fertilization can take place around 48 hours before ovulation or 24 hours after.

During the menstrual cycle, when the graafian follicle ruptures and an ovum is ejected from the ovary into the fallopian tube, that ovum is encased by a thick membranous envelope called the zona pellucida. This thick membranous envelope along with the corona radiata (elongated follicle cells surrounding the zona pellucida of the ovum) are captured by the fimbriated edges of the fallopian tube and propelled into the tube. Once inside the fallopian tube, hairlike structures that line the inside of the fallopian tube move the ovum through the tube along with peristaltic action.

During sexual intercourse the male ejaculates about 200 to 500 million sperm (a teaspoon of semen) into the vagina. The cervical mucus during ovulation is much less viscous and hostile to sperm, thereby aiding spermatozoa greater penetration. Using the flagellar movement of their tails, the sperm can fertilize an ovum in 5 minutes or in 4 to 6 hours, which is usually the average time. Sperm remain viable in the reproductive tract for 2 to 3 days. The great number of sperm ejaculated is necessary because many sperm are lost in the vagina, cervical mucus, and endometrium or enter the wrong fallopian tube, which does not contain an ovum.

A prerequisite for a sperm to fertilize an ovum is called capacitation, which is an enzymatic process. Small perforations form in the cap of the sperm (acrosome) which allows hyaluronidase (an enzyme) to help the sperm penetrate the protective layers (corona radiata and zona pellucida) around the ovum. Only one sperm can fertilize one ovum in normal circumstances. Once the sperm penetrates the ovum’s cell membrane,
the cell membrane changes and becomes impenetrable to other spermatozoa, and this process is called the zona reaction.

During the fusion of the ovum and the sperm, cell division begins and the structure is called a zygote. The sex of the embryo is determined at the time of fertilization because the ovum and sperm each carry 23 chromosomes (22 autosomes and 1 sex chromosome); therefore the fertilized ovum has 46 chromosomes. A male child is conceived if a Y-carrying spermatozoon fertilized the ovum. This child will have received an X chromosome from the mother and a Y chromosome from the father, XY. A female child is conceived if an X-carrying spermatozoon fertilized the ovum. This child will have received an X chromosome from the mother and an X chromosome from the father, XX.

After fertilization the ovum passes through several cell divisions, during which it is retained in the fallopian tube for about 3 days. Fertilization takes place (day 1), and the fertilized ovum divides and is called a zygote (day 2). As cell division continues, a cluster of cells form, called the morula (day 3). The morula is still surrounded by the protective cell covering, called the zona pellucida. This cluster of cells continues to travel down the fallopian tube until it enters the upper aspect of the uterus. At this point large cells collect at the periphery of the ball with a fluid space surrounding an inner cell mass; this phase is called the blastocyst (day 4). As fluid passes through the zona pellucida, it is eventually shed by the growing structure. The blastocyst is divided into two parts: The inner surface of the blastocyst forms the embryo and amnion and the outer layers of cells surrounding the blastocyst cavity are called the trophoblasts. Eventually, the trophoblast develops into one of the embryonic membranes, the chorion, and helps to form the placenta.