Week 2
At 12 days, the embryo is still just a clump of around a couple of thousand cells. The different parts of its body do not start to grow until after the first two weeks. Now completely embedded into the wall of the womb, some cells are starting to form the placenta, which will bring the mother's blood vessels right alongside those of the baby. These cells begin to produce a hormone, which can be detected by a pregnancy test.
