This week your little one is about a quarter of an inch long and about the size of a lentil bean. Her heart will start beating around 22-23 days after fertilization which is the first organ to develop. This little embryo has circulating blood and if you have a 6-week ultrasound the doctor or technician might be able to locate the fetal heartbeat (though week 7 increases these odds).
While your pregnancy is still in its infancy (pun intended), and your little embryo is small, there are big things happening and a lot of development in the works. The foundation for your baby's respiratory (lungs), hepatic (liver), and optic (eye) systems are all starting. The thyroid and pituitary glands are also taking shape and the folds of tissue near the "top" of the embryo will develop into a jaw, cheeks, and chin, with small spots where his eyes and a nose will appear in a few weeks. Also busy developing are ear canals, brain, muscles, and bones.