Up to 22 weeks
By this time your baby has grown enough to have developed a nervous system and muscles that allow him to move around in your womb. Because he’s still so small, he can swim up and down and be in any position at any time.
Your Baby’s Progress
Up until about 19 weeks after your last period, your baby grows very rapidly. Now this growth rate slows down, apart from his weight gain, and he matures in other ways. He begins to build up his defense systems.
A sheath begins to form around the nerves in his spinal cord to protect them from possible damage. He also has his own primitive immune system, which will help to defend him from some infections. To make body heat and keep up his temperature, your baby needs some specialized fatty tissue. This is provided by a substance called “brown fat,” which began to form during the fourth month. Now, deposits of brown fat begin to build up in areas of his body such as his neck, chest, and crotch. This will continue until term. One of the reasons that premature babies are so vulnerable is that they don’t yet have enough brown fat, and so cannot keep themselves warm.
His skin will continue to grow, although it’ll be red and wrinkled because there’s so little fat underneath it. His body begins to get plumper from now on. The sebaceous glands (oil-producing glands in the skin) become active and make a waxy, greasy substance called vernix caseosa. This protects his skin during its long immersion in the amniotic fluid.
Your baby’s body is also covered with fine hair called lanugo. Nobody is quite sure why babies have this hair, but it may help to regulate the body temperature, or it may help hold the protective vernix caseosa in place.
His movements As his nerve fibers become connected and his muscles continue to develop and grow stronger, his movements become more purposeful and coordinated. He embarks on his own gymnastics program-stretching, grasping, turning-to build up his muscles, improve his motor ability, and strengthen his bones. These movements can make your abdomen sore.
Sex organs A boy’s scrotum is solid at this stage. A girl’s vagina starts to become hollow and her ovaries contain about seven million eggs, which will be reduced to about two million at birth. By the time she reaches puberty, she’ll have between 200,000 and 500,000 eggs, and she’ll release only 400-500 of these during her adult life-around one per month until menopause. Nipples and underlying mammary glands develop in both sexes.
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