How Does a Baby Grow in the Womb?

In roughly forty weeks, a healthy, fertilized egg becomes a baby. For many years, fetal development was a mystery hidden within a woman's body. But with medical advances and high-tech cameras, we now know a great deal about how a baby develops in the womb.

  1. The First Month

    • By the time an embryo makes its way into the womb and attaches itself to the uterine lining, it has developed from a single egg into an embryo complete with all of its DNA information. The authors of What to Expect When You're Expecting describe the embryo as "tadpole-like" at this stage, smaller than even a grain of rice, but point out how quickly the tiny mass of cells is developing. By the end of the first month the baby has a head with a mouth opening, a basic heart and the beginning of a brain.

    The First Trimester

    • Throughout the first trimester (conception through three months), a baby grows at an astonishing rate, turning from a tiny group of cells into a fetus about the size of peach. The typical baby at thirteen weeks of pregnancy is between 2.5 and 3 inches in length, crown-to-rump . A fetus draws nutrition from its mother, so it's important that pregnant women eat healthy food at regular intervals to nourish their growing babies.

    The Second Trimester

    • During the second trimester (four to six months), the baby continues to grow rapidly, reaching 9 inches crown-to-rump by month seven. The uterus at this point is roughly the size of a soccer ball. Because skin is developing at a faster rate than fat, the baby's skin will be loose, red and wrinkled. Babies also grow short, fuzzy hair on their bodies called lanugo, which will disappear during the last trimester. The baby's internal organs are maturing and he is able to swallow amniotic fluid and urinate. The baby now has a fully formed face, and some babies have even been photographed sucking their thumbs in the womb during the second trimester.

    The Third Trimester

    • The baby continues to gain weight rapidly during the last trimester (seven to nine months). By the end of the seventh month, the average baby weighs around 3 pounds and measures about ten inches crown-to-rump . The lanugo begins to disappear at this point, while hair starts to grow on the baby's head. Around seven months the baby has eyebrows and eyelashes. The baby's brain develops rapidly during the last trimester, which is just one of the reasons premature labor is not preferable. The lungs also mature near the end of pregnancy, though a baby who is born early is still likely to survive because of advanced medical technology.

    The Last Month

    • Over the last few weeks of pregnancy, a baby nears his birth weight and length. Her skull is not fully solid, which will help in the birthing process. As the baby moves through the birth canal, his head is slightly flexible and able to shift without harming the baby's brain. A few weeks after birth, his head will again be round.

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