By 24 to 26 weeks, the fetus is fully-formed, in the sense that all of the main organs are there and working, although not terribly efficiently. Babies born at this gestational age now have a good chance of survival (which wasn't the case just a few years ago!), but it takes tremendous effort and skill on the part of neonatologists, nurses, and other specialists to make that happen. In the uterus, during the last 12-13 weeks (the third trimester), the main business is growth.

Growth depends on the placenta

The main event for the baby during the last weeks of pregnancy is growth. Her weight triples in the last 12 weeks, and her length doubles. She takes on stores of fat, protein (in the form of muscles and other tissues), calcium (bones), and key nutrients such as iron.

All of this build-up depends on a well-functioning placenta. In the placenta, blood from the mother circulates around a system of tiny fetal blood vessels, allowing nutrients and oxygen to pass from the mother's blood to the baby's.

Because the fetus and mother are genetically different, blood cannot simply flow from the mother into the baby. Instead, nutrients from the mother's blood drift through the walls of the fetal capillaries in the placenta and enter the fetal bloodstream. Waste products from the baby pass in the opposite direction.

The placenta is one of the first structures to form during pregnancy, but it really comes into its own in the last trimester. Illnesses or environmental forces which interfere with the placenta can restrict the fetus' growth.

One such factor is cigarette smoking by the mother, which causes blood vessels to squeeze shut. In the placenta, that means less blood available to the infant--and thus less nutrition. Severe diabetes in the mother also can interfere with the blood vessels in the placenta, and hence with infant growth.

Brain growth

The most dramatic growth takes place in the brain with new brain cells and many new connections between cells forming by the minute. The long root-like extensions called axons carry electrical signals from one part of the brain to another, some over quite long distances. A fatty substance, called myelin, develops around the axons, allowing the electrical signals to move much more rapidly and efficiently.

A lot of the brain growth in the third trimester (and in the first two to three years of life) involves laying down of myelin. Myelin is also important in the spinal cord and the nerves that reach to every part of the body, allowing the quick back and forth flow of information that is required for coordinated, skillful movement. The jerky, poorly coordinated movements of newborns (and particularly in premature infants) are due in large part to their relative lack of myelin.

Can I come out, yet?

The official definition of "full term" is 38 to 42 weeks of gestation. From 36 weeks on, most infants do well, with minimal special care. At younger and younger gestational ages, more medical intervention is necessary, along with greater chances of long-term problems relating to the prematurity. Still, with modern care many very tiny, very early babies thrive. Given a choice, however, the best place for most babies during the third trimester is inside.

Remember your baby's first kick? Or was that just your tummy rumbling? Last month your baby's movements were gentle flutters. By the end of this month she'll be much stronger. You won't have any doubt that your little one is on the move.

Your baby is beginning her rapid weight gain now. Her muscles are developing. So are her motor skills. Her bones are storing calcium and getting harder. Her rolls, tumbles, kicks and pokes may have the power to startle you. Especially when you look down at your belly and catch sight of your baby's "calisthenics."

Some babies are more active than others. Some are active in daylight, others at night. Ask your doctor if you have questions about the timing, frequency or intensity of your baby's movements.

Last month your baby became aware of sounds inside and outside the womb. This month, she will get used to certain sounds and stop reacting to them. This process is called habituation. It's one of the simplest, earliest forms of memory.

Let's say you live on a busy street. Your baby was startled when she heard car horns blaring for the first time. But now she can learn to "tune it out." She'll refine this skill after birth. With luck she'll sleep soundly through traffic jams and much more.

Your baby is surrounded by amniotic fluid, not air. Do you wonder how she will breathe at birth?

During pregnancy, you breathe for your baby. Her lungs won't function until she's born. But she's making preparations now to breathe on her own. Her lungs will begin to make surfactant this month. This fatty substance will cover the inner lining of the air sacs in the lungs. This allows them to expand more easily. So when your newborn needs to take her first breath, she'll be ready.

By the end of this month, your baby will weigh 1-1/2 to 2-1/2 pounds. She'll also begin sleeping and eating at regular intervals.

Check out the progress of Marc's work in the kitchen by going to the Crib section of the site!

Your Baby: The Beginning Of Braxton Hicks

Your growing fetus weighs in at around 1 pound and measures approximately 10 to 11 in length. Her body is well proportioned, but it still slender, with little fat. She's also sporting new developments from head to toe: her fingernails are almost completely developed; meconium, the baby's first bowel movement, is forming in her intestines; and the lanugo that covers her body is darkening. You're probably feeling a lot of movement and may even notice some early, relatively painless contractions, called Braxton Hicks. Don't worry -- these contractions don't mean you're going into labor, and your baby is unaffected by them.

• Proportions of the body are now quite similar to a newborn although thinner since she hasn't begun to form body fat.
• Bones located in the middle ear harden.
• Your baby is able to hear. (Dad, did you know? Low-frequency sounds mimicking a male voice penetrate the abdomen and uterine wall better than the higher frequencies of the female voice?)
• The eyes are formed, though the iris still lacks pigmentation.
• The pancreas, essential in the production of hormones, is developing steadily. She has begun producing insulin, important for the breakdown of sugars.
• If born now, your baby has a 15% chance of survival, her odds going up with each passing day. .
• The average baby at this stage weighs 1.1 pound (501gm) and is 11.38 inches (28.9cm) long.

Your baby is really starting to look like a baby now! Now the fetus's senses are becoming more defined with taste buds being complete. Additionally, the nerve endings have been developed enough at this point for the fetus to feel things using her hands. If you could watch, you would see more movement and even to the extent of feeling other parts of her body. The baby has complete eyebrows and eyelids now.

Some other facts include:

• Your baby weighs close to a pound at this point!
• Your baby can now hear your conversations more clearly than before!. When you talk, read, or sing, expect her to hear you. Studies have found that newborns will suck more vigorously when read to from a book they heard frequently in utero.
• Eyelids and eyebrows are fully formed.
• Fingernails have grown to the end of the fingers.
• Be prepared for all those "Why" questions coming! Your baby's brain has entered a stage of rapid growth, especially in what's called the germinal matrix. This structure deep in the middle of the brain serves as a kind of factory for brain cells and disappears shortly before birth. But the brain's amazing expansion program continues until around the fifth birthday.
• With some help from mom, baby's liver is starting to break down bilirubin, a substance produced by red blood cells.
• Length is 10.94 inches (27.8cm); weight is nearly 1 pound (430gm).

With 22 weeks down and 18 more to go, it's time to start thinking about labor and delivery. Check out this signs of labor article to learn the ins and outs (and do's and don'ts) of the birthing process.

Sources:
[University of Pennsylvania Health Center]
[Pregancy.org]