Your developing baby is just a tiny ball of hundreds of cells (called a blastocyst) which are multiplying like mad! It contains a fluid-filled chamber which will become the amniotic sac, and an inner cell mass that will develop into the fetus.
An outer group of cells will form the placenta, which provides vital oxygen and nutrients to your baby and removes waste products. These cells have started to produce a hormone called HCG (human chorionic gonadotropin) which sends a signal to stop your ovaries releasing more eggs and keeps your uterus from shedding its lining. You can now say goodbye to your period for the next nine months!