What Happens in Each of the Trimesters of Baby Development?

When a baby is conceived, the development of the child begins a wild, 40-week journey to what is called a "full-term" pregnancy. These 40 weeks are divided into three equal periods, referred to as trimesters. Each trimester lasts about three months. In each of these trimesters, the development of the baby passes several milestones.

First Trimester

• You can begin to count the weeks of your pregnancy from the first day of the last menstrual period you had. About two weeks after that, the egg was fertilized, and the developing baby, called a zygote, began to divide itself before implanting in the uterus.

  During the week after your missed period, you can see the developing baby on an ultrasound, even though it measures about one-fifth of an inch in diameter. One week later, the heartbeat can be seen on the ultrasound.

  Human-like features begin to take shape in the seventh through 10th weeks of pregnancy. The baby's head, body and appendages are visible in the ultrasound.

  As you near the end of the first trimester, the baby will be about 3 inches in length.

Second Trimester

• In the beginning of the second trimester, your baby's body begins to become more proportional. The head doesn't grow as quickly as the body, so the body catches up to what was a larger head.

  Facial details still can't be seen on an ultrasound, but the baby now is able to make such expressions as squinting, frowning or showing a grimace. You might even see your baby suck her thumb during an ultrasound.

  Most mothers will have an ultrasound performed for the first time during their second trimester, normally around the 20th week of pregnancy. The check is done to see how the baby is developing. If the baby cooperates, you might even be able to learn whether you are having a boy or a girl.

  At this point, your baby is about 5 or 6 inches long, weighing about half a pound.

  Near the middle of the trimester, she can even pick up some outside sounds, and she can hear her mother's voice.

  As you end the trimester, your baby has all but mastered the art of swallowing -- a skill she will need for life outside the womb. Babies are active at this stage of pregnancy, and you might feel like you are being beaten up from the inside.

  At the 25th week of pregnancy, the baby is about 12 inches long and weighs about a pound. An ultrasound will show more detail in her facial features.

Third Trimester

• Growth happens quickly during the first half of the third trimester. Your baby will go from about 1.5 lbs. to nearly 3.5 lbs. in just six weeks. He will also grow about 3 inches in length in these six weeks, growing from 14 inches to about 17 inches.

  Your baby's internal organs are maturing during this time period. One of the most important organs that is completing development at this time are his lungs. Your own tummy may be taking hard hits because his muscles are gaining strength. This means his kicks can really pack some power.

  In your last seven weeks of pregnancy, your baby is growing by about a pound a week. At the 36th week, the average child will weigh about 6 lbs. and be about 19 inches long.

  At 37 weeks of pregnancy, your child is considered full term. You may not go into labor, though, for some time.

Beyond the Third Trimester

• If you are still pregnant at 40 weeks, you are considered past your due date. Although few women deliver on their actual due date, you will likely find that fact to be of little comfort if you past the 40-week milestone.

  At 40 weeks gestation, infants weigh about 7.5 lbs. and measure about 20 inches long. If you go beyond 40 weeks, your baby will continue to put on weight at a rate of about an ounce a day.

  Your doctor may decide to induce labor if you do not go into labor on your own in the 41st or 42nd week.