Hello,
The process of development begins on the day a woman’s egg is fertilized by a sperm which has penetrated it. The fertilized egg or
zygote immediately begins to divide into 2 cells, then 4, then 8, and so on. After a few days the cluster of between 13 and 32 cells leave the fallopian tube and migrate into the uterus. This group of cells is called a blastocyst and the size has increased to hundreds of cells.