Fetal Development Weeks 36 Through 40

See what's happening with your baby in the ninth month of your pregnancy.

pregnant woman at doctors office
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Week 36

For a child about to be born, baby fat is a very good thing. Your baby is working hard to accumulate all the fat he can at this point -- his body is growing rounder, and those adorable dimples are forming at elbows and knees. This build-up of fat -- about 15 percent of his total weight--will help him maintain his body temperature and give him a store of energy. Because the fat beneath his skin is white in color, it makes his skin appear lighter and less ruddy.

In a first-time mother, the baby often "drops" two to four weeks before delivery, as the baby's head descends into the mother's pelvic cavity. This is called "engagement," or "lightening," and it means it won't be long now before the big day!

Week 37

The finish line is finally within reach! At the end of this week, your baby is considered full term. That means that no special precautions will be taken to prevent labor if it begins -- from here on in, it's all systems go. Your baby is still squirming in her crowded quarters. Up until now, she's been gaining about a half-pound a week, but now growth slows to an ounce or so a week. That's a good thing, because almost all the available space in the uterus is filled up.

Through most of the pregnancy, the baby has relied on her mother for immunities against infections, but in the past few weeks, her own immune system has begun to develop. This process will continue after birth, and can be helped along by encouraging the baby to breast-feed. In the first days after birth, mom's breasts produce "colostrum," a thin fluid that fortifies the baby's immune system; the breast milk that follows is also chock-full of immunity-building benefits.

Week 38

Big baby or little one? The average full-term newborn weighs between 6 and 9 pounds and is 18 to 21 inches long. Your doctor will probably be able to give you an idea of your baby's size at this point, but be aware that sometimes these are only predictions -- no one will know baby's precise size until birth. As you watch for signs that your waters have broken (contrary to what you've seen in the movies, this big event precedes labor only about 10 percent of the time), remember that these "waters" are actually amniotic fluid, the solution protects your baby from infection. At the end of the pregnancy, you're carrying between 1 to 2 pints of this precious, life-sustaining liquid.

About two weeks before birth, the placenta starts to deteriorate just a bit. It becomes less efficient in transferring nutrition, and blood clots and calcified patches start to show. This vital but short-lived organ is reaching the end of its life span.

Week 39

Only five percent of babies decide to greet the world on their due date, so yours could arrive at anytime now. While you're waiting, mull over this word of the week: fontanels. These are the two soft spots on a baby's head where the skull bones don't yet join together. The fontanel allows the flexible skull bones to shift and bend as baby journeys through the birth canal, making for an easier trip without damaging the brain. Don't panic if your little one's head looks elongated or even cone-shaped right after birth -- the bones will quickly remold themselves into a rounded shape. During the course of baby's first year, the skull will harden, and by 18 months, the fontanels will have disappeared.

At the time of birth, the baby has a total of 300 bones. Some of these bones will fuse together as the baby grows; adults have a total of 206 bones.

Week 40

The day you've been waiting for is here at last! Of course, your baby doesn't know that it's your due date--she may decide to linger in her cozy uterine home up to two more weeks. But whatever birthdate she chooses, she'll be bringing along a bag of tricks: a full-term baby can display more than 70 different reflexes, the automatic and unlearned behaviors needed for survival. One such behavior is crying, a sound you're sure to hear very soon. But don't expect to see tears right away: babies are born with an underdeveloped tear duct system. Tears don't usually show up until 1 to 3 months of age.

The mother's partner in the birthing room is often allowed to cut the umbilical cord after delivery. This lifeline between placenta and fetus may be up to 4 feet long at birth. After the cord is cut, a remnant will remains attached to the baby's abdomen. It will drop off sometime in the first month, leaving behind a tiny belly button, a permanent reminder of those precious nine-plus months when mother and child were bonded together as one.

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