It's difficult to imagine, but this week your baby is still tinier than the "I" at the start of this sentence. Even so, exciting changes are taking place each day. Your embryo now looks less like a ball and more like a curled tube. One end of it will eventually become your baby's head; the other, your baby's bottom. Between those two ends, the spinal cord is forming, and by the 5th week there will be a series of bumps along your baby's back. This is the start of your baby's central nervous system.
Like a baby chick, your baby has a pear-shape yolk sac filled with fluid that will help nourish him. Blood moves to the wall of the yolk sac by your baby's primitive heart, where it circulates and then is returned. Your baby's heart is only a small tube now, but by next week it will have right and left chambers.
Other major organs are also developing rapidly. Your baby's brain has divided into two lobes by the start of this 2nd month of life, and his respiratory system will first appear as a groove behind his face. It will keep growing downward as your baby develops, branching 23 times along both sides of his body as the lungs finish forming. The arms and legs are tiny buds, and minuscule, paddle-shape hands may even show finger buds. Your baby will finally be visible to the naked eye.
Originally published in You & Your Baby: Pregnancy.
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