Fertilization usually occurs in the middle section of the tube, not in the uterus, when one sperm penetrates the outer layer and then passes through another layer, attaching to the surface and losing its tail. The membranes of the sperm and egg unite, causing the egg to react by releasing an enzyme so no other sperm can enter. The genetic material from you and your partner are combined, each supplying 23 chromosomes. Once this happens, you are most definitely pregnant!
At the moment of conception, your baby’s gender is determined already. Out of the 46 chromosomes that make up your baby’s genetic material, two of them (one from your partner’s sperm and the other from your egg) determine your baby’s sex. If the sperm that penetrated your egg has a X chromosome, your baby will be a girl and if the sperm has a Y chromosome, your baby will be a boy.
After fertilization, the egg divides and multiplies into several cells. It continues this constant division as it travels from your fallopian tube towards your uterus, until it forms a solid ball of cells. When it arrives in your uterus, after a 1- 1 1/2 week journey, it already consists of over 250 cells. Next, it attaches to the lining of your uterus (the endometrium) and implants itself, while it continues it’s rapid growth. The fertilized egg is now considered a “blastocyst”. It is made up of two layers that eventually will become the placenta and the embryo.