At 28 weeks, your baby measures about 14 inches (about 35 cm) and weighs about 2-1/2 pounds (about 1,135 grams). But by the end of the third trimester — at 40 weeks, your due date — it measures about 20 inches (50 cm) and weighs 6 to 8 pounds (about 2,700 to 3,600 grams) — sometimes a bit more, sometimes a bit less.

The fetus spends most of the third trimester growing, adding fat, and continuing to develop various organs, especially the central nervous system. The arms and legs get chubbier, and the skin becomes thicker and smooth.

During the third trimester, your baby is less susceptible to infections and to the adverse effects of medications, but some of these agents may still affect its growth. The last two months are usually spent getting ready for the transition to life in the world outside the uterus. The changes are less dramatic than they were early on, but the maturation and growth that happen now are very important.

By 28 to 34 weeks, the fetus generally assumes a head-down position (called a vertex presentation). This way, the buttocks and legs (the bulkiest parts of its body) occupy the roomiest part of the uterus — the top part. In about 4 percent of singleton pregnancies, the baby may be positioned buttocks-down (breech) or lie across the uterus (transverse).

[Credit: Kathryn Born, MA]
Credit: Kathryn Born, MA

By 36 weeks, growth slows, and amniotic fluid volume is at its maximum level. After this point, the amount of amniotic fluid may start to decline because blood flow to the baby's kidneys decreases as the placenta ages, and the baby produces less urine (and therefore less amniotic fluid).

In fact, most practitioners routinely check the amniotic fluid volume on ultrasound or by feeling your abdomen during the last few weeks to make sure that a normal amount remains.

Movin' and shakin': Fetal movements

Look down at your belly during times of fetal activity during the third trimester, and it may appear that an alien from outer space is doing an aerobic dance inside you. Although fetal movements don't actually diminish as your due date approaches, the timing and quality of the movements change.

Toward the end of pregnancy, fetal movements may feel less like jabs and more like tumbles or rolls, and you notice longer periods of quiet between movements. The fetus is adapting to a more newborn-like pattern, taking longer naps and having longer active cycles.

If you don't sense a normal amount of activity, let your practitioner know. A good general rule is that you should feel about six movements in one hour after dinner, while resting. Any movement, no matter how subtle, counts. Some women find that they go for periods of feeling less fetal movement, but then the movements pick up again and are normal.

This is very common and isn't a reason to be concerned. However, if you notice a pattern of diminishing fetal movements or you feel absolutely no fetal movements over several hours (despite resting or eating), give your practitioner a call right away.

If you have certain risk factors or if you need specific guidelines to track the adequacy of fetal movements, your practitioner may suggest that you keep a diary to chart fetal movement, starting at 28 weeks. You can track fetal movements in several different ways.

One way is to lie down on your left side after dinner to count fetal movements, and write down how long it takes to count ten movements. Another way of doing the test is to count fetal movements while lying down for an hour each day (it doesn't have to be the same hour every day) and to plot the number of movements on a chart given to you by your practitioner.

With this method, you can see the pattern of the baby's movements throughout the day.

Flexing the breathing muscles

Fetuses undergo what are called rhythmic breathing movements from 10 weeks onward, although these movements are much more frequent in the third trimester. The fetus doesn't actually breathe, but its chest, abdominal wall, and diaphragm move in a pattern that is characteristic of breathing.

You don't notice these movements, but a doctor can observe them with ultrasound. These movements are signs that the baby is faring well. During the third trimester, the amount of time a fetus spends performing the breathing movements increases, especially after meals.

Hiccupping in utero

At times, you may feel a quick, rhythmic pattern of fetal movements, occurring every few seconds. These movements are most likely hiccups. Some women feel fetal hiccups several times throughout the day; others sense them only rarely.

Occasionally you may actually see the baby hiccupping during an ultrasound exam. These hiccups are completely normal. They may feel strange, but standing on your head and drinking water, which we hear is a great cure, probably isn't your best option now.