5 Weeks

Your baby's microscopic heart begins to beat this week—although it won't be detectable with one of those cool Dopplers for a few weeks, so you'll have to wait a little longer to hear that satisfying "whoosh-whoosh" you're so anxious for. Here's what else is under construction: With the beating of the heart, blood will begin to circulate throughout the body. Your baby's first organ system will be up and running!The beginnings of the brain; the cardiovascular, nervous and reproductive systems; as well as all other major systems are under way. Your baby's new digs—the amniotic sac and the placenta (aka the hotel and the restaurant)—are still forming at this point. (Maybe you should send a housewarming gift.)All that high-speed development and your baby is only 2 millimeters long—about the size of a sesame seed (check one out next time you're chowing down on a bagel—you won't believe how tiny it is!).