Fetal development

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16-week fetus	24-week fetus	26 to 28-week fetus	3.5 week fetus
30 to 32 week fetus	8.5 week fetus	Fetus (10 weeks old)	Fetus (12 weeks old)
Fetus (7.5 weeks old)

The end of the eighth week marks the end of the "embryonic period" and the beginning of the "fetal period".

• Weeks 9 to 12
  • the fetus reaches a length of 3.2 inches
  • the head comprises nearly half of the fetus' size
  • the face is well formed
  • eyelids close and will not reopen until about the 28th week
  • tooth buds appear for the baby teeth
  • limbs are long and thin
  • the fetus can make a fist with its fingers
  • genitals appear well differentiated
  • red blood cells are produced in the liver
• Weeks 13 to 16
  • the fetus reaches a length of about 6 inches
  • a fine hair develops on the head called lanugo
  • fetal skin is almost transparent
  • more muscle tissue and bones have developed, and the bones become harder
  • the fetus makes active movements
  • sucking motions are made with the mouth
  • meconium is made in the intestinal tract
  • the liver and pancreas produce their appropriate fluid secretions
• Week 20
  • the fetus reaches a length of 8 inches
  • lanugo hair covers entire body
  • eyebrows and lashes appear
  • nails appear on fingers and toes
  • the fetus is more active with increased muscle development
  • "quickening" usually occurs (the mother can feel the fetus moving)
  • fetal heartbeat can be heard with a stethoscope
• Week 24
  • the fetus reaches a length of 11.2 inches
  • the fetus weighs about 1 lb. 10 oz.
  • eyebrows and eyelashes are well formed
  • all the eye components are developed
  • the fetus has a hand and startle reflex
  • footprints and fingerprints forming
  • alveoli (air sacs) forming in lungs
• Weeks 25 to 28
  • the fetus reaches a length of 15 inches
  • the fetus weighs about 2 lbs. 11 oz.
  • rapid brain development
  • nervous system developed enough to control some body functions
  • eyelids open and close
  • respiratory system, while immature, has developed to the point where gas exchange is possible
  • a baby born at this time may survive, but the possibilities for complications and death remain high
• Weeks 29 to 32
  • the fetus reaches a length of about 15-17 inches
  • the fetus weighs about 4 lbs. 6 oz.
  • rapid increase in the amount of body fat
  • rhythmic breathing movements occur, but lungs are not fully mature
  • bones are fully developed, but still soft and pliable
  • fetus begins storing iron, calcium, and phosphorus
• Week 36
  • the fetus reaches a length of about 16-19 inches
  • the fetus weighs about 5 lbs. 12 oz. to 6 lbs. 12 oz.
  • lanugo begins to disappear
  • increase in body fat
  • fingernails reach the end of the fingertips
  • a baby born at 36 weeks has a high chance of survival, but may require some medical interventions
• Weeks 37 to 40
  • considered full-term at 37 weeks
  • may be 19 to 21 inches in length
  • lanugo is gone except for on the upper arms and shoulders
  • fingernails extend beyond fingertips
  • small breast buds are present on both sexes
  • head hair is now coarse and thicker

Review Date: 05/10/2006
Reviewed By: Daniel Rauch, MD, FAAP, Director, Pediatric Hospitalist Program, Associate Professor of Pediatrics, NYU School of Medicine, New York, NY. Review provided by VeriMed Healthcare Network.