Your Developing Baby: Week 3

The genetic material for your developing baby comes from two sources: the mother's egg and the father's sperm. Each of these cells contains 23 chromosomes, so your baby will have a total of 46. With this genetic blueprint in place, your baby can start to form. And from those early beginnings, much of your baby-to-be's physical appearance, gender, and even her personality traits are decided. But it will take weeks (around 40!) for your baby to be fully developed.

The coming together of the egg and the sperm, called fertilization, most often takes place in the Fallopian tube. The fertilized egg then begins to divide rapidly into groups of cells. One group of cells will form a gestational sac to protect the other group of cells, which will become the embryo.

A fertilized egg moves down the Fallopian tube and into the uterus over several days, usually between 6-12. Once inside the uterus, the fertilized egg implants itself along the uterine lining. Most likely, you will not feel when implantation occurs, although you might notice some cramping or even light spotting. At the time of implantation, the placenta also begins to develop, but as with the tiny baby-to-be, it will take several weeks for the placenta to fully form. Eventually the placenta will connect through the umbilical cord from Mother to Baby.

Implantation: In human reproduction, the point at which the fertilized egg attaches itself to the uterine wall in the very earliest stages of pregnancy.

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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).