The embryo in the fifth week of pregnancy is usually only initially recognizable as a placenta and body tip. The period of ten weeks of gestation known as the embryo is a time of organ differentiation and formation, and during the fifth week of pregnancy the initial fetal brain sac forms and the heartbeat begins to appear. The embryo is about 0.2 centimeters long and is the size of a large grain of rice. In the fifth week of pregnancy, i.e. 35 days, for women with a menstrual cycle of about 28 days on ultrasound you can usually only see the fetal sac, you cannot yet see the formation of the yolk sac or the fetal buds, nor can you hear the fetal heart. For women with long menstrual cycles, this may not show up on ultrasound at this time, and blood HCG, or human chorionic gonadotropin, is needed to detect it.