The third week of gestation coincides with the first month of pregnancy, but the woman has not yet had the menstrual delay and does not know that she is pregnant.
On the other hand, the embryo has only one week of embryonic development. It has just been conceived and only consists of a few cells. It is not yet called a fetus because it has not been implanted in the uterus. It is also not possible to visualize it with an ultrasound because its size is minuscule and the gestational sac has not yet formed.