A Trimester by Trimester Guide to Fetus Development

Find out what happens to the baby inside your womb during each trimester.
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First Trimester

Here's a month-by-month guide to your baby's development:

Month 1

  • Tiny limb buds, which will grow into arms and legs, appear.
  • The embryo looks like a tadpole.
  • The heart and lungs begin to form. By the 25th day, the heart starts to beat.
  • The neural tube, which becomes the brain and spinal cord, begins to form.
  • At the end of the first month, the embryo is about 1/2 inch long and weighs less than 1 ounce.

Month 2

  • All major body organs and systems are formed but not completely developed.
  • The early stages of the placenta, which exchanges nutrients from your body for waste products produced by the baby, are visible and working.
  • The ears, ankles, and wrists form. The eyelids form and grow but are sealed shut.
  • Fingers and toes develop.
  • By the end of the second month, the fetus looks more like a person than like a tadpole, is about 1 inch long and still weighs less than 1 ounce.

Month 3

  • After 8 weeks as an embryo, the baby now is called a "fetus."
  • The fingers and toes have soft nails.
  • The mouth has 20 buds that will become "baby teeth."
  • You can hear your baby's heartbeat for the first time (10 to 12 weeks) using a special instrument called a Doptone.
  • For the rest of pregnancy, the body organs will mature, and the fetus will gain weight.
  • By the end of this month, the fetus is 2 1/2 inches long and weighs a little over 1 ounce.

Second Trimester

Month 4

  • The fetus moves, kicks, and swallows.
  • The skin is pink and transparent.
  • The umbilical cord continues to grow and thicken to carry enough nourishment from mother to fetus.
  • The placenta is fully formed.
  • By the end of the fourth month, the fetus is 6 to 7 inches long and weighs about 5 ounces.

Month 5

  • The fetus becomes more active, turning from side to side and sometimes head over heels.
  • The fingernails have grown to the tips of the fingers.
  • The fetus sleeps and wakes at regular intervals.
  • The fetus has a month of rapid growth. At the end of the fifth month, the fetus is 8 to 12 inches long and weighs 1/2 to 1 pound.
  • By the end of the fifth month (20 to 21 weeks), fetal activity can be felt by the mother.

Month 6

  • The skin is red and wrinkled and covered with fine, soft hair.
  • The eyelids begin to part and the eyes open.
  • The finger and toe prints can be seen.
  • The fetus continues its rapid growth. At the end of the sixth month, the fetus is 11 to 14 inches long and weighs 1 to 1 1/2 pounds.
  • If born at 24 weeks or more, the fetus might survive with intensive care.

Third Trimester

Month 7

  • The fetus can open and close its eyes and suck its thumb.
  • The fetus exercises by kicking and stretching.
  • The fetus responds to light and sound.
  • If born now, the fetus has a good chance for survival.
  • The fetus is now about 15 inches long and weighs about 3 pounds.

Month 8

  • Rapid brain growth continues.
  • The fetus is too big to move around much but can kick strongly and roll around.
  • You may notice the shape of an elbow or heel against your belly.
  • The bones of the head are soft and flexible to make it easier for the baby to fit through the birth canal.
  • The lungs may still be immature. If born now, before 37 weeks, the fetus is premature but has an excellent chance for survival.
  • The fetus is now about 18 inches long and weighs about 5 pounds.

Month 9

  • At 37 to 40 weeks, your baby is full-term.
  • The baby's lungs are mature and ready to work on their own.
  • During this month the baby gains about 1/4 to 1/2 pound a week.
  • The baby moves into position to be born, usually dropping into a head-down position and resting lower in the mother's pelvis.
  • By the end of the ninth month, the baby weighs 6 to 9 pounds and is 19 to 21 inches long.

Source: March of Dimes

Reviewed 11/02 by Elizabeth Stein, CNM

All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.

American Baby

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