Section 3.1 Prenatal development

This stage of human development begins with conception and ends when the zygote is implanted in the wall of the uterus. It typically lasts for the first two weeks after conception. At the moment of conception, two single cells, one from a male (sperm) and one from a female (ovum), merge to form a new cell called a zygote.

After conception, the zygote begins its journey, spending about a week floating down the fallopian tube toward the uterus. Cell division begins approximately 24 to 36 hours after fertilization. Within two to three days, the multiplying cells form a mass that contains several dozen cells and is roughly the size of a pinhead.

Around four days after conception, this mass of cells is referred to as a blastocyst, which starts to take a spherical shape with two layers of cells surrounding a hollow center. The outer layer of the blastocyst will develop into structures that support the growing organism, such as the placenta, while the inner layer will eventually become the embryo.

The final step of this stage is implantation, during which the blastocyst attaches itself to the wall of the uterus. This process is usually complete within 10 to 14 days after conception, by which time the blastocyst contains about 150 cells and is ready to begin the next stage of prenatal development.