Women & Children's Services at Valley Health

Fetal Development

Fetal Development

Fetal development during the first trimester:

 

The most dramatic changes and development occur during the first trimester. During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester it becomes a fetus that is fully formed, weighing approximately 1/2 to one ounce and measuring, on average, three to four inches in length.

 

Fetal development during the second trimester:

 

Now that all the major organs and systems have formed in the fetus, the following six months will be spent growing. The weight of the fetus will multiply more than seven times over the next few months, as the fetus becomes a baby that can survive outside of the uterus.

By the end of the second trimester, the fetus will be about 13 to 16 inches long and weighs about 2 to 3 pounds. Fetal development during the second trimester includes the following:

  • The fetus kicks, moves, and can turn from side to side.

  • The eyes have been gradually moving to the front of the face and the ears have moved from the neck to the sides of the head. The fetus can hear the mother's voice.

  • A creamy white substance (called vernix caseosa, or simply vernix) begins to appear on the fetus and helps to protect the thin fetal skin. Vernix is gradually absorbed by the skin, but some may be seen on babies even after birth.

  • The fetus is developing reflexes such as swallowing and sucking.

  • The fetus can respond to certain stimuli.

  • The placenta is fully developed.

  • The brain will undergo its most important period of growth from the 5th month on.

  • Fingernails have grown on the tips of the fingers and toes, and the fingers and toes are fully separated.

  • The fetus goes through cycles of sleep and wakefulness.

  • Skin is wrinkly and red, covered with soft, downy hair (called lanugo).

  • Hair is growing on the head of the fetus.

  • Fat begins to form on the fetus.

  • Eyelids are beginning to open and the eyebrows and eyelashes are visible.

  • Fingerprints and toeprints have formed.

  • Rapid growth is continuing in fetal size and weight.

  • The 20th week marks the halfway point of the pregnancy.

Fetal development during the third trimester:

During the third trimester, the fetus continues to grow in size and weight. The lungs are still maturing and the fetus begins to position itself head-down. By the end of the third trimester, the fetus is about 19 to 21 inches long and weighs, on average, six to nine pounds. Fetal development during the third trimester includes:

  • The fetus can see and hear.
  • The brain continues to develop.
  • The kidneys and lungs continue to mature.
  • By the 36th week, the head may "engage" (drop into the pelvic area) - a process called "lightening."
  • The bones of the skull remain soft to make it easier to pass through the birth canal.
  • For many babies, the irises of the eyes are slate blue. The permanent eye color will not appear until several days or weeks after birth.
  • The fetus can suck its thumb and has the ability to cry.
  • By 38 to 40 weeks, the fetus' lanugo has disappeared almost completely.
  • By 38 to 40 weeks, the lungs have matured completely.
  • The baby is covered in vernix caseosa (or simply called vernix), a creamy, protective coating on the skin.
  • The head will usually turn downward during the last couple of weeks of pregnancy.

Click here for more information on what to expect during each trimester