The most critical period of its development is between three and seven weeks after fertilization, when a simple heart tube assumes
the shape
of a four-chambered heart. In fact,
the heart actually begins beating by
the 22nd day
of life.
When you are 5 weeks pregnant, your baby's heart is made up of two tiny channels called heart tubes and they're already hard at work, beating to their own drummer (it will be weeks before those beats become coordinated).