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Fetal Development Guide

The Development of Your Unborn Baby Month By Month

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, watch the The Miracle of Life video and review the book A Child is Born by Lennart Nielson for a pictorial review of your baby's development.