Placental membranes release the hCG hormone, human chorionic gonadotropin. This hormone is involved until the 12th week and acts on the ovary, stopping the period and producing progesterone; sickness and dizziness are caused by the gonadotropin. Estrogens and progesterone are also produced: they are very important during the pregnancy, because they control immune system, milk production and uterus and breast growing.
hCG hormone is present in urines and can be detected through a pregnancy test, for this reason it is possible that in this third week you could already have the positive result of your pregnancy.
During this third week of pregnancy you could feel mild abdominal pains, which indicates the growth of your uterus, and headache. You could also notice that your breast is bigger and more sensitive than normal, that your are sleepy and need to urinate more often. You could feel tired, sick and with an increased sensitivity to odors.
After the second week, the embryo is already implanted in the uterus; between the third and the fourth week the gastrulation begins, the process through which 3 germinative layers are formed: endoderms, mesoderms and ectoderms.