In the 3rd week of pregnancy the sperm meets the egg and fertilization occurs. Your baby, just a tiny ball with hundreds of cells, will now begin developing in leaps and bounds after it has settled in the uterus.
Your body begins to secrete pregnancy hormones, and the placenta develops. These hormones send a signal to stop ovulation, and ensure the protection of the internal layer of the uterus and of its little guest by triggering the increase of oestrogen and progesterone hormones, which start placental growth. By the end of this week, you can take a pregnancy test, due to the presence of the HCG hormone.