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Countdown to Baby

So you're pregnant! Now what? The next nine months will be unlike any you've ever experienced. At first, you may not notice the transformations taking place. But soon enough, you'll see burgeoning evidence of all the changes going on inside. Many of them will be thrilling, others unnerving. It's downright strange -- and miraculous -- to wake up every day wearing a body that's slightly different from the one you went to sleep in.

While people tend to think about pregnancy only in terms of trimesters, each week brings about new developments in both you and your baby-to-be. Here's a look at the unfolding wonders of pregnancy that can help you tune in to the miracle within.

Weeks 1 to 12

Week 1 This is the week of your last menstrual period before pregnancy. You won't conceive for another two weeks, but the first day of significant bleeding is considered the official start date of pregnancy perhaps because it's an easier date to recognize than the date of conception.

Week 2 In anticipation of new life, the uterus forms a blood-rich lining of tissue called the endometrium. At the same time, in one of the two ovaries, eggs ripen in fluid-filled sacs called follicles.

Week 3 Around midcycle (day 14 of the typical 28-day cycle), one of the eggs is swept into a fallopian tube. This is ovulation. If in the next 24 hours one of the 350 million sperm in the average ejaculate can trek all the way from the vagina through the uterus and to the fallopian tube to penetrate the egg, bingo -- fertilization. Sperm can live in the tube awaiting a mature egg for one to five days, so the lovemaking that created your baby could have taken place before ovulation. The fertilized egg immediately closes its outer membrane to other sperm and begins dividing into a cluster of identical cells as it floats down the fallopian tube to the uterus. If you are going to have twins, either one fertilized egg will split, creating identical twins, or two eggs will be separately fertilized, creating fraternal twins.

Week 4 The fluid-filled cluster nests inside the uterus, where it divides in two. The half attached to the uterine wall becomes the placenta; the other half will become the fetus. By the end of this week you'll miss a period, though you may experience staining called implantation bleeding.

Week 5 The ball of cells, about the size of an apple seed, has become an embryo. The placenta and umbilical cord, through which the baby will receive nourishment and oxygen, are on the job. This is when many women first suspect pregnancy; a home test can often confirm it. Schedule a visit to your doctor -- most first prenatal checkups take place between six and ten weeks -- and ask about taking a folate supplement, even before the appointment: Weeks five through ten are critical to neural development.

Week 6 The embryo looks more like a tadpole than a human. Its heart, no bigger than a poppy seed, has started beating. Major organs are developing, and the neural tube, which connects the brain and spinal cord, closes. You may begin to experience the physical sensations of pregnancy -- nausea, breast tenderness, fatigue, frequent urination. At your first prenatal visit, your doctor should test you for rubella immunity, Rh factor, and sexually transmitted diseases.

Week 7 The embryo is the size of a raspberry. Its head is large in proportion to the body. Dark spots where the eyes and nostrils will be, pits that mark the ears, and protruding buds that will become the arms and legs are beginning to form.

Week 8 The embryo has distinct slightly webbed fingers and toes and see-through-thin skin. Your uterus is now the size of a small orange.

Week 9 The strawberry-size being is now a "fetus" and is constantly moving, though you won't feel it yet. What you probably will notice is that you're spilling out of your old bra and need better support.

Week 10 In both shape and size, the fetus resembles a medium shrimp. Already, its genitals are beginning to form, though your doctor can't yet tell the sex by looking at a sonogram.

Week 11 The fetus, about two inches long and less than half an ounce, is swallowing and kicking. Its vital organs are in place and each day more minute details fill in. The rapid "whoosh" of its heartbeat can be heard through a special stethoscope. Your uterus is the size of a grapefruit.

Week 12 Most likely, your nausea begins to wane now, and energy picks up. The uterus moves from the pelvic floor to front-and-center in your abdomen, relieving pressure on your bladder. Now about two and a half inches long, the fetus is fully formed, from tooth buds to toenails. With the most critical development past, your chances of carrying and delivering a healthy baby greatly increase.

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.

Weeks 29 to 40

Week 29 The fetus is not quite 12 inches long and weighs between 2 and 3 pounds -- but it will double or triple in weight between now and birth. As space gets tighter, less acrobatic tumbling takes place, though you'll still feel plenty of stretching and kicking.

Week 30 The fetus has eyelashes and any hair with which he'll be born. An overwhelming majority of babies born at 30 weeks survive -- over 90 percent -- and about 60 percent of these preemies will grow up without any long-term health problems or disabilities.

Week 31 Though the scenery in the womb sure isn't Paris, the fetus can discern light and dark and blink its eyes. Now is the time to interview pediatricians, preregister at the hospital, and write your birth plan. This outlines what you would ideally like to take place during your delivery, including whether or not you'll receive an epidural, which birthing positions you'd like to try, and whether you'd like a son to be circumcised.

Week 32 A layer of fat is forming beneath the thin, wrinkly fetal skin. Review your childbirth-class notes and practice your breathing and relaxation. Your doctor may do another glucose screen in the next couple of weeks.

Week 33 The fetus is exercising its lungs by practicing breathing -- inhaling amniotic fluid. You're gaining a pound a week now; roughly half of that goes right to the fetus. In fact, your baby-to-be gains 50 percent of its birth weight during the next seven weeks.

Week 34 Most babies settle into the head-down position, although it may not be final. The skull bones are still quite pliable and not completely joined to ease the exit through the birth canal. Pack your hospital bag now -- better to be early than sorry you forgot something.

Week 35 Your obstetrician will probably check your cervix weekly until you deliver. You'll be tested for Group B streptococcus bacteria between weeks 35 and 37; if you test positive, you will most likely be given an antibiotic during childbirth to protect your baby from infection. Be sure to tell your doctor if you are allergic to penicillin. Ninety-nine percent of babies born at this point survive, most with no major problems. Not only are the lungs more developed, but, thanks to advances in neonatal care, respiratory difficulties -- once a leading killer of preemies born before 35 weeks -- are much more readily overcome.

Week 36 Your uterus has expanded to a thousand times its original volume and now reaches up to the base of your rib cage. The baby may drop lower in your abdomen, the head engaging within the pelvic bones at the birth canal.

Week 37 By the end of this week, your pregnancy has come full term; the baby could be born any day. More good news: Your weight gain has probably hit its peak.

Week 38 Most of the fetus's downy lanugo and waxy vernix have disappeared, although some may remain at birth. They get swallowed by the fetus along with other secretions in the amniotic fluid and lodge in the baby's bowels. They'll become the child's first bowel movement, a dark-green, tar-like waste called meconium.

Week 39 The average full-term newborn is 20 inches long and weighs 7 to 7½ pounds. Boys tend to be slightly heavier than girls. Take it easy; if you aren't inclined to do much, don't.

Week 40 Don't fret if your baby isn't born by your due date -- just 5 percent hit the mark exactly. Most doctors wait two weeks before considering a pregnancy overdue. But it won't be long before the miracle that nature started so many weeks ago is finally nestled in your arms.

From the PARENTING Guide to Pregnancy & Childbirth, by Paula Spencer with the editors of PARENTING magazine.