You may not feel any different, but huge changes are taking place in your body. Chemical signals are sent to stop your periods and the placenta produces the hormone human chorionic gonadotropin (hCG). Around 12 days after fertilisation, hCG levels will be high enough to give a positive pregnancy test.
After fertilisation, the egg and sperm fuse to create one cell called the zygote. This divides to form a cluster of cells that travels down the fallopian tube to the uterus. Three days later the cell cluster, or blastocyst, implants itself in the uterus wall and becomes the embryo and part of the placenta.