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).