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Countdown to Baby

Weeks 13 to 28

Week 13 The fetus's head is still big in relation to the body. It will squirm if your abdomen is gently prodded.

Week 14 The fetus is about as big as a large goldfish. Baby's facial features and fingerprints are already set. Amniocentesis is performed, if needed or desired, between now and 18 weeks. Women with Rh-negative blood get an Rh immunoglobulin shot at the time of the amnio.

Week 15 The fetus's skin is covered with lanugo, fuzzy down that usually disappears before birth. Hair and eyebrows begin to grow. A blood test called the triple screen (it measures alpha fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol) is scheduled between 15 and 20 weeks.

Week 16 Most women need maternity clothes by now. Although you've probably gained a few pounds, the fetus accounts for less than three ounces. Much of the weight gain is due to your blood volume, breasts, placenta, uterus, and amniotic fluid.

Week 17 All systems are go: The fetus's lungs begin to exhale amniotic fluid, the circulatory system is operating, and the urinary tract is up and running.

Week 18 Most mothers-to-be start to feel their baby move (a phenomenon called quickening) between 16 and 20 weeks. First-time moms may not recognize this until week 24. The fetus is about five inches long and weighs nearly five ounces.

Week 19 An ultrasound is often done around 15 to 22 weeks to assess fetal growth and development, screen for some defects, check the placenta and umbilical cord, and verify the due date. During this sneak preview you may see the baby-to-be kick or even suck its thumb. If the fetus is in the right position, it's possible to determine the sex.

Week 20 The top of your uterus now reaches your belly button and will grow about one centimeter each week. The baby weighs approximately nine ounces and is around six and one half inches long. You're halfway there!

Week 21 The fetus is steadily gaining fat and has grown a coat of a waxy, whitish substance called vernix to protect its skin during the lengthy submersion in amniotic fluid. Now is the time to look into childbirth-education classes.

Week 22 Your growling stomach, beating heart, and a distorted version of your voice form the background music for your baby-to-be's life in the womb. Loud, sudden noises may cause the fetus's heart rate to rise and its limbs to flail.

Week 23 At about one pound, your future baby is proportioned like a newborn, though still scrawny. You, however, are steadily gaining weight. Although the survival rate for babies born now is only about 20 percent, the chances for a healthy baby grow exponentially each day.

Week 24 The fetus is the size of a box turtle, and its hearing is well established. You may be given a glucose screening to detect gestational diabetes between now and 28 weeks.

Week 25 If you feel repeated blips in your midsection that bring to mind a steadily dripping faucet, that's the baby having hiccups.

Week 26 The fetus weighs about one and a half pounds and is roughly nine inches long. A preemie's chance of survival jumps to 75 percent by this point.

Week 27 As your uterus expands to accommodate all the growth, you may see stretch marks on your belly. Most women have gained 16 to 22 pounds by this point.

Week 28 If born early, the baby (by now 11 to 14 inches long, at a weight of just over two pounds) will probably survive in a neonatal intensive-care unit, with a reasonably good chance of no serious complications. Rh-negative women should get another Rh immunoglobulin shot, even if they've already received a shot after amniocentesis.