Although your baby is a mere 1/2 inch long and still curved as tightly as a comma, he's growing quickly. Some of the most important growth is happening in his brain, where new nerve cells are forming at the astounding rate of 100,000 per minute. As your baby's brain cells multiply, they will branch out and connect, forming the first primitive pathways for his central nervous system. By the time your baby is born, his brain will have more than 10 million intertwined nerve cells!
By now, your baby's heart rate is even faster than it was last week, and his heart tube has formed small bulges. His cells are frantically churning out the building blocks for what will become 40 pairs of muscles and 23 pairs of vertebrae radiating from his spinal column. Your baby's limb buds are starting to grow; they look like tiny flippers at this point. His outer ears are beginning to take shape, and he now has joints in his elbows, wrists, and knees. His eyelids are nearly complete too.
Originally published in You & Your Baby: Pregnancy.
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