The First Two Weeks of Pregnancy

Even before a woman has a clue she might be pregnant, an amazing array of events are taking place in her body. From the moment of conception, when one lucky sperm beats out tens of millions of others to meet up with and fertilize the egg that was released during that month's ovulation, the clock begins ticking on what will be a remarkable 40 weeks or so of the development of a new human being.

Here's a look at what's going on behind the scenes during the first two weeks of pregnancy—the germinal stage.

A New Life's First Journey

When an egg is fertilized, the result is a single-celled organism called a zygote, which then develops into a multicellular structure called an embryo. Almost immediately the zygote is on the move, making its way from the Fallopian tube where fertilization took place toward the uterus. It can take as long as a week for the zygote to complete the trip.

The Embryonic Phase in Prenatal Development

Meanwhile, the zygote is already beginning to change. Within 24 hours to 36 hours of fertilization, the zygote will begin to divide and grow in a process called mitosis. One cell will become two cells; two cells will become four cells; four cells will become eight cells; eight cells will become 16 cells; and so on. (About 50% of embryos studied in a laboratory don't make it beyond these first few rounds of cell division.)

At the eight-cell mark, the multiplying cells will begin to differentiate. This means each one will take on certain characteristics that will determine the type of cell it eventually will become—a skin cell, for example, or a lung or kidney cell.

As the cells multiply, they also will separate into two layers: the inner layer will eventually develop into a fetus, the outer layer will become the placenta. By day 5 post-fertilization, the mass of cells will have become what's called a blastocyst.

Womb At Last

When the blastocyst arrives at its final destination, the next step is for the outer layer of cells to implant themselves into the walls of the uterus. They'll do this by burrowing into the lining of the uterus, rupturing tiny blood vessels as they nestle in. A web of blood vessels and membranes, the placenta, will form.

Implantation is not always an automatic and sure-fire process. But when implantation is successful, the hormonal changes that will eventually produce symptoms of pregnancy will begin to take place.

Most symptoms begin approximately one week after implantation (or 26 to 30 days from the last menstrual period). At this point, a home pregnancy test will undoubtedly have a positive result and a new life will be well on the way to birth.