Main content

    Fetal Development

    The growth of your baby is a continuous process that often conveys a sense of awe. The following information will help you understand your baby's rapid growth and highlights milestones in development. (Sizes and weights are approximate).

    First Trimester

    At the end of 4 weeks:

    • Embryo is 1/4 inch in length


    • Heart, digestive system, backbone and spinal cord begin to form


    • Placenta (sometimes called "afterbirth") begins developing


    • The single fertilized egg is now 10,000 times larger than size at conception
    At the end of 8 weeks:
    • Embryo is 1-1/8 inches in length


    • Eyes, nose, lips, tongue, ears and teeth are forming


    • Penis begins to appear in boys


    • Embryo is moving, although the mother cannot yet feel movement


    • Heart is functioning
    At the end of 12 weeks:
    • Fetus is 2-1/2 to 3 inches long


    • Weighs about 1/2 to 1 ounce


    • Nails start to develop and earlobes are formed


    • Fetus develops recognizable form


    • Arms, hands, fingers, legs, feet and toes are fully formed


    • Eyes are almost fully developed


    • By this stage, a fetus has developed most of his/her organs and tissues


    • Fetal heart rate can be heard at 10 weeks with a special instrument called a Doppler

    Second Trimester

    At the end of 4 months:

    • Fetus is 6-1/2 to 7 inches long


    • Weighs about 6 to 7 ounces


    • Fetus is developing reflexes such as sucking and swallowing. Fetus may begin sucking his/her thumb


    • Tooth buds are developing


    • Sweat glands are forming on palms and soles


    • Fingers are well defined


    • Sex is identifiable


    • Skin is bright pink, transparent and covered with soft, downy hair


    • Although recognizably human in appearance, the baby would not be able to survive outside the mother's body
    At the end of 5 months:
    • Fetus is 8 to 10 inches long


    • Weighs about 1 pound


    • Hair begins to grow on his/her head


    • Soft, woolly hair called lanugo will cover baby's body (and some may remain until a week after birth when it is shed)


    • Mother begins to feel fetal movement


    • Internal organs are maturing


    • Eyebrows, eyelids and eyelashes appear
    At the end of 6 months:
    • Fetus is 11 to 14 inches long


    • Weighs about 1-3/4 to 2 pounds


    • Eyelids begin to part and eyes open occasionally for short periods of time


    • Skin is covered with protective coating called vernix


    • Fetus is able to hiccup

    Third Trimester

    At the end of 7 months:

    • Fetus is 14 to 16 inches long


    • Weighs about 2-1/2 to 3-1/2 pounds


    • Taste buds have developed


    • Fat layers are forming


    • Organs are maturing


    • Skin is still wrinkled and red


    • If born at this time, he/she will be considered a premature baby and require special care
    At the end of 8 months:
    • Fetus is 16-1/2 to 18 inches long


    • Weighs about 4 to 6 pounds


    • Overall growth is rapid this month


    • Tremendous brain growth occurs at this time


    • Most body organs are now developed with the exception of the lungs


    • Movements or "kicks" are strong enough to be visible from the outside


    • Kidneys are mature


    • Skin is less wrinkled


    • Fingernails now extend beyond fingertips
    At the end of 9 months:
    • Fetus is 19 to 20 inches long


    • Weighs about 7 to 7 1/2 pounds


    • Skin is pink and smooth


    • The lungs are mature


    • Baby is now fully developed and can survive outside mother's body


    • He/She settles down lower in the abdomen in preparation for birth and may seem less active
    For further information, review the video The Miracle of Life and the book A Child is Born by Lennart Nielson for a pictorial review of your baby's development. Both are available at Newborn Connections.