Fetal Development Weeks 9 Through 13

See what's happening with your baby in the third month of your pregnancy.

Week 9

Sometime in the last few days, the first detectable brain waves went coursing through your baby's head. What kind of thoughts might she be thinking? She's nearing an inch in length, about a big as your pinky toe.

Boy or girl? The answer to pregnancy's biggest question was actually settled seven weeks ago, at the moment the egg was fertilized when the sperm contributed either a Y chromosome, which would create a boy, or an X chromosome, which makes a girl. But until this point in its development, the embryo has had no outward signs of gender. This week, the gonads, or reproductive glands, come into being. They are the tiny beginnings of either testes or ovaries. But don't rush to get your ultrasound just yet- those tiny sex organs won't be visible for another six to eight weeks.

The baby's muscles are growing and getting stronger, and its movements are increasing. Elbows, knees, and other limb joints are distinguishable. Any day now, toenails will appear at the tips of budding toes.

Week 10

Hooray! Your baby is almost ready to graduate! He's entered his final stage of embryonic development at this point, and within the next week he'll make the big leap from "embryo" to "fetus." All of his essential internal and external structures are in place; in other words, he has all of the same body components that you do. He's over one and one-quarter inches long now, or about the length of your index finger from its tip to the first joint.

The intestines, which had been developing inside the umbilical cord, have now begun slipping into the embryo's abdominal cavity. Genitalia are still nearly impossible to see by ultrasound.

Baby's tail has disappeared by now. His arms are stubby now, and his two hands can almost touch each other as they move in front of his bulging belly. The ears are almost fully developed, both on the outside of the head and in the inner ear. Taste buds are dimpling the surface of a miniscule tongue. Eyelids lay half-closed close over tiny eyes, which already have some of the color they'll have at birth.

Week 11

Take a moment to celebrate with your baby this week: you've both reached a milestone. Risks of serous malformations drop sharply at this point in your pregnancy, and so do risks of miscarriage. The major work of generating organs, limbs, and other body parts is mostly finished. At this point, your baby has made the transition from embryo to fetus. Afloat in about 1.5 ounces of warm amniotic bathwater, he's now about as long as a paper clip -- and a lot more interesting! In less time than an average school semester, your baby has grown from a single-celled organism to a complex being made up of millions of differentiated cells. Now the process is less about bringing body parts into being and more about allowing those parts to grow and develop.

Each finger and toe is clearly visible now -- no more ducklike webbing -- and teeth are growing in tiny tooth sockets. The teeth will remain hidden at birth, only to poke out of baby's gums as some point in the first year, leading to those first adorable toothy grins and some nasty teething pains, too.

Week 12

By this week, your baby is, on average, about two-and-a-half inches long (keep in mind that, after week 11, growth rates can vary a lot), and could be cradled snugly in a soup spoon. Her brain's basic structure is now in place, and brain mass is growing all the time.

Several important digestive functions begin around this week. The pancreas begins to produce insulin, a hormone that regulates blood sugar levels. The liver starts to secrete bile, a thick greenish fluid stored in the gall bladder that helps the body digest fats.

Though still not quite "cute," the fetus is starting to look more like a person now. The bridge of the nose and the jawbones are starting to create the familiar outlines of a human face. Inside the mouth, folded tissues come together to make the palate, or roof of the mouth. Fingernails emerge from their nail beds. These nails can grow quite long; more than a few babies are born needing a quick manicure.

Week 13

Welcome to the final week of the first trimester! By the end of this week, your baby is about three inches in length, and would fit cozily into the cupped palm of your hand. Although you can't feel his movements, he's all over the place at this point, arms and legs waving, body wriggling. Sometimes he gets the hiccups. Though he may seem agitated, he's really just feeling his oats, testing his brand-new powers of motion while he still has the room -- by the end of the next trimester, his living space will be much more cramped.

External sexual organs are beginning to emerge now, with clitoris and labia developing in girls, penis and scrotum in boys, but these organs still aren't detectable by ultrasound.

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Reviewed 2008

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