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. If the egg implants in the fallopian tube, you will have an ectopic pregnancy but this is a rare occurrence. More commonly, the embryo cruises down the fallopian tube just fine, growing into a solid ball of about 200 cells by the time it enters your uterus about four days after conception. The embryo usually bounces around in the uterus for a few days until it finds a suitable spot to implant in the lining. 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.
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.
Ectopic pregnancy: A fertilized egg growing outside the uterus. Most often ectopic pregnancies develop in the Fallopian tube, where the egg travels from the ovary to the uterus. This condition poses a serious health risk to the mother, requiring the health care provider to remove the fertilized egg and end the pregnancy.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).