This week your baby-to-be's bones continue to develop. You might notice that they've become stronger and more solid each time he gives you a quick jab in the ribs. That's because your baby now weighs well over 2-1/2 pounds and most likely measures about 15 inches long. He's beginning to store enough fat beneath his lovely pink skin that he may already have those baby dimples on his elbows and knees.
While your baby's bones do mature in utero, it's important that they stay flexible to allow for more growth not only in the womb, but also once your baby is born. A baby's bones continue to grow and harden well into childhood. Flexible bones are also necessary for delivery. The bones on your unborn baby's head are made up of large pieces. These cranium plates aren't fused together, but they come together toward the center and top part of your unborn baby's forehead. You might have heard some people refer to this as a newborn's "soft spot." Because these skull pieces aren't fused together, but can move, the baby's head can more easily be pushed through the birth canal. If the unborn baby's skull were hard and immovable, it would be impossible to deliver a baby through the relatively small opening in the cervix — even though it does stretch to 10 centimeters!
At week 28, your baby's hearing is so well developed by now that he will startle at loud noises. Soon he'll start turning his head to listen to sounds outside your body.
As she runs out of room, your baby may bow her head down toward her knees, curling as she did when she was very tiny. She's long enough now that you'll notice her kicks more. Forget those little fluttery motions. This feels more like someone is elbowing you for space on the subway! Play "guess that body part" with your partner as you try to distinguish a knee from a heel.
Anterior fontanel: The gaps where the bones come together on the skull of an unborn baby (which can be observed on a newborn) are called the fontanels. The largest, the anterior fontanel, is a ?soft spot? that looks almost like a diamond shape under the skin on your baby's forehead. The bones in the cranium will eventually fuse together sometime during the first 12 -- 18 months of your baby's life.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).