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Human Development in the Womb

Human development consists of approximately 40 weeks, or nine months, a fetus develops into a baby. For example, in the first to fourth weeks, conception begins. The fertilized egg called the zygote moves to a women's uterus where it becomes a blastocyst and implants onto the uterine wall. The blastocyst spends the rest of the time in a womb undergoing two phases of development: embryonic and fetal periods.

  1. First Trimester

    • The first trimester involves the embryonic period of human development. During the three months, or 12 weeks, a fetus rapidly develops from cells to a distinguishable fetus. For example, in week five, a fetus begins to form a spinal cord, neck, toes, organs and brain. Also, an embryo's heart begins pumping blood and facial features starts forming. By the end of the first trimester, the fetus weighs approximately ½ oz. or 14 grams and is 2 ½ inches (61 millimeters) long.

    Second Trimester

    • During the second trimester, human development builds on what happened during the first three months of pregnancy. For instance, although a baby's ears develop in the first trimester, he can't hear until the second trimester. During the second trimester a baby's skeleton forms, body hair grows and fingerprints and footprints appear. Also, at week 16 facial muscles develop allowing a baby to frown, squint and make sucking motions. Parents can learn the sex of a baby and the mother feels a baby move. After the second trimester, a baby weighs about two lbs. or 820 grams and is 9 inches or 230 millimeters long.

    Third Trimester

    • In the last trimester of human development, a baby prepares for delivery. For instance, in week 30 a baby opens her eyes and in week 32 practices breathing. Sexual development also continues throughout the trimester. A baby typically weighs approximately 7½ lbs. or 3,400 grams and is about 14 inches (360 millimeters) long.

    Considerations

    • The first trimester is a critical time for a fetus. For example, alcohol, certain prescription and recreational drugs, and other substances can cause birth defects. An infection like rubella and nutritional deficiencies can also damage the fetus during the first trimester. Undergoing radiation therapy or X-rays affects a fetus during the first three months of a pregnancy.

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