Lets go back to conception.
An hour after fertilization the nuclei of the ovum and the sperm fuse, forming a zygote . For the first 3 to 4 days after fertilization, the zygote stays in the fallopian tube. Here it undergoes rapid mitotic divisions forming a ball of cells called the morula. Implantation does not occur for another three to four days. While the morula is floating about the uterus many changes are occurring.