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Fetus Development in Humans

The first trimester of pregnancy is active with the drastic development of your new baby's vital organs and bone structure. It's an important time to take extra care of yourself and maintain a healthy regimen. There will be a lot of growth and development in the next nine months as that fetus grows and matures into your special bundle of joy.

  1. First Trimester

    • The first trimester begins with the formation of the heart and brain of the fetus. It's also a time for the multiplication of its blood cells. The fetus will immediately begin to pull nutrients from the mother's blood stream. By the end of the eighth week, the fetus has grown to about a ½-inch; it has buds for the arms and legs. Each organ is developing. Eyelids, hair and facial features are taking shape.

    Two to Three Months

    • A fetus has a heart that beats up to 155 beats per minute after two months of development. The head is nearly half the size of the fetus, and its body will move involuntarily. The nervous system is forming and connecting to the brain; the brain itself is still developing. The fetus has defined limbs. The genitals are forming at this point, although the baby's sex cannot yet be determined.

    Second Trimester

    • The second trimester begins at 15 weeks. The baby's skin is now forming, and he can make facial expressions. Fats have begun to be stored under the skin. Your baby is also beginning to hear around 18 weeks, and he may be startled by loud noises. A layer of hair is covering the fetus.

    20 Weeks

    • The fetus is half-developed at 20 weeks. It's approximately12 inches long. Bone marrow is creating blood cells, and the brain is working with the nerves to develop the senses of taste, touch and hearing. Your baby will begin to grow hair and eyebrows at this stage.

      Around week 24, your baby's lungs will produce a substance that allows the air sacks in the lungs to inflate. The inner ear is complete, and this gives balance to your child. The baby is about two pounds now, and he looks more like a baby than a fetus.

    Third Trimester

    • At week 26 of development, your baby would have an 85 percent chance of survival if born early. At this stage, the baby is fattening up and storing nutrients. The hair covering your baby is beginning to disappear, and he is about 17 inches long.

      The baby may begin to turn downward in preparation for birth. All organs are developed. Fingernails and toenails have begun to grow.

    Nine-months

    • The average newborn weighs around seven pounds. On average, he's approximately 19 inches long. Fetal movement has been significantly reduced, primarily because the baby has no more room in your tummy. It's time for the baby to come into the world.

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