WHAT TO EXPECT

Michael Sawyer, Hour 2, September 13, 2013

First Trimester

Before your baby actually starts growing, you set the stage. Last week an increase in the amount of estrogen and progesterone coursing through your bloodstream prompted your uterus to form a lush, blood-rich lining of tissue to support a potential fertilized egg. At the same time, in your ovaries, eggs were ripening in fluid-filled sacs called follicles. At the beginning of this week (often around day 14 of a 28-day cycle), you ovulate: One of your eggs erupts from its follicle and is swept away from your ovary and into a Fallopian tube.

During the next 12 to 24 hours, that egg will be fertilized if one of the 250 million sperm (on average) your mate ejaculates manages to swim all the way from your vagina through your cervix, up into your uterus to the Fallopian tube, and penetrate the egg. Only about 400 sperm will survive the arduous ten-hour journey to the egg, and only one will succeed in burring through its outer membrane. (It takes about 20 minutes for the lucky winner to find its way in.)

This week marks the beginning of the embryonic period. From now until 10 weeks, all of your baby's organs will begin to develop and some will even begin to function. As a result, this is the time when she'll be most vulnerable to anything that might interfere with her development. Right now your baby is an embryo the size of a poppy seed, consisting of two layers: the Epiblast and the hypo-blast, from which all of her organs and body parts will develop. The primitive placenta is also made up of two layers at this point. Its cells are tunneling into the lining of your uterus, creating spaces for your blood to flow so that the developed placenta will be able to provide nutrients and oxygen to your growing baby when it starts to function at the end of this week.

Fingerprints have formed on your baby's tiny fingertips, her veins and organs are clearly visible through her still-thin skin, and her body is starting to catch up with her head — which makes up just a third of her body size now. If you're having a girl, she now has more than 2 million eggs in her ovaries. Your baby is almost 3 inches long (about the size of a pea pod) and weighs nearly an ounce.

Second Trimester

This week's big developments: Your baby can now squint, frown, grimace, pee, and possibly suck his thumb! Thanks to brain impulses, his facial muscles are getting a workout as his tiny features form one expression after another. His kidneys are producing urine, which he releases into the amniotic fluid around him — a process he'll keep up until birth. He can grasp, too, and if you're having an ultrasound now, you may even catch him sucking his thumb. In other news: Your baby's stretching out. From head to bottom, he measures 3 1/2 inches — about the size of a lemon — and he weighs 1 1/2 ounces. His body's growing faster than his head, which now sits upon a more distinct neck. By the end of this week, his arms will have grown to a length that's in proportion to the rest of his body. (His legs still have some lengthening to do.) He's starting to develop an ultra-fine, downy covering of hair, called lanugo, all over his body. Your baby's liver starts making bile this week — a sign that it's doing its job right — and his spleen starts helping in the production of red blood cells. Though you can't feel his tiny punches and kicks yet, your little pugilist's hands and feet (which now measure about 1/2 inches long) are more flexible and active.

Your growing baby now measures about 4 inches long, crown to rump, and weighs in at about 2 1/2 ounces (about the size of an apple). She's busy moving amniotic fluid through her nose and upper respiratory tract, which helps the primitive air sacs in her lungs begin to develop. Her legs are growing longer than her arms now, and she can move all of her joints and limbs. Although her eyelids are still fused shut, she can sense light. If you shine a flashlight at your tummy, for instance, she's likely to move away from the beam. There's not much for your baby to taste at this point, but she is forming taste buds. Finally, if you have an ultrasound this week, you may be able to find out whether your baby's a boy or a girl! (Don't be too disappointed if it remains a mystery, though. Nailing down your baby's sex depends on the clarity of the picture and on your baby's position. He or she may be modestly curled up or turned in such a way as to "hide the goods.")

Get ready for a growth spurt. In the next few weeks, your baby will double his weight and add inches to his length. Right now, he's about the size of an avocado: 4 1/2 inches long (head to rump) and 3 1/2 ounces. His legs are much more developed, his head is more erect than it has been, and his eyes have moved closer to the front of his head. His ears are close to their final position, too. The patterning of his scalp has begun, though his locks aren't recognizable yet. He's even started growing toenails. And there's a lot happening inside as well. For example, his heart is now pumping about 25 quarts of blood each day, and this amount will continue to increase as your baby continues to develop.


Third Trimester

By this week, your baby weighs 2 1/4 pounds (about the size of a large eggplant) and measures 14.8 inches from the top of her head to her heels. She can blink her eyes, which now sport lashes. With her eyesight developing, she may be able to see the light that filters in through your womb. She's also developing billions of neurons in her brain and adding more body fat in preparation for life in the outside world.

Your baby now weighs about 2 1/2 pounds (about the size of a butternut squash) and is a tad over 15 inches long from head to heel. His muscles and lungs are continuing to mature, and his head is growing bigger to make room for his developing brain. To meet his increasing nutritional demands, you'll need plenty of protein, vitamins C, folic acid, and iron. And because his bones are soaking up lots of calcium, be sure to drink your milk (or find another good source of calcium, such as cheese, yogurt, or enriched orange juice). This trimester, about 250 milligrams of calcium are deposited in your baby's hardening skeleton each day.

Your baby's very active now. Your healthcare provider may ask you to spend some time each day counting kicks and will give you specific instructions on how to do this. Let your provider know if you ever notice that your baby is becoming less active. You may need a non-stress test or biophysical profile to check on your baby's condition. Some old friends — heartburn and constipation — may take center stage now. The pregnancy hormone progesterone relaxes smooth muscle tissue throughout your body, including your gastrointestinal tract. This relaxation, coupled with the crowding in your abdomen, slows digestion. Sluggish digestion can cause gas and heartburn — especially after a big meal — and contribute to constipation.

Your baby's about 15.7 inches long now and weighs almost 3 pounds (about the size of a large cabbage). A pint and a half of amniotic fluid surrounds her, but that volume will shrink as she gets bigger and takes up more room in your uterus. Her eyesight continues to develop, though it's not very keen; even after she's born, she'll keep her eyes closed for a good part of the day. When she does open them, she'll respond to changes in light but will have 20/400 vision — which means she can only make out objects a few inches from her face. (Normal adult vision is 20/20.)

About 5 to 6 percent of women have prolonged pregnancies that extend three or more weeks beyond their estimated due dates. Babies born at 42 weeks and beyond can have dry parchment-like skin and are often overweight. Waiting that long to deliver also increases your chance of developing an infection in your uterus that could be dangerous for your baby or of having a stillbirth. What's more, your labor is more likely to be prolonged or stalled, both you and your baby have an increased risk of injury during a vaginal delivery, and you double your chances of needing a C-section.