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Weeks of Pregnancy: What to Expect

The entire process of pregnancy-from conception to birth-takes about nine months, or 40 weeks of pregnancy. The pregnancy is divided into three-month periods known as trimesters.

The First Trimester (Weeks 1 to 13)

The first three months of fetal development are, perhaps, the most important. During the first 60 days, most of the baby’s organs form. It is at this stage that the unborn child is most sensitive to environmental chemicals, drugs and viruses that can cause birth defects.

The average fetus is about 3 inches long and weighs about 1 ounce by the end of the first trimester. It is normal for the fetus’ head to be disproportionately larger than the rest of its body. Some other important developments during the first trimester include:

  • By week 7, it is usually possible to see the developing baby within the womb and detect its heartbeat by ultrasound examinations.
  • By week 8, the baby’s face and features begin forming.
  • The first bone cells form.
  • Fingers and toes are growing, along with the beginnings of nails.
  • The liver begins making bile (a liquid that helps break down fats in food so they can be absorbed), and the kidneys begin secreting urine into the bladder.
  • The circulatory and respiratory systems begin functioning.
  • The fetus also begins to move during the first trimester, although the mother won’t feel movement until around the fourth month.

The Second Trimester (Weeks 14 to 26)

During this time, the fetus begins to grow and its organs mature. The increasing size of the uterus becomes obvious in the second trimester of pregnancy, as the woman’s belly begins to swell. Many women need to start wearing looser or maternity clothes at this time.

In the womb, a protective layer of amniotic fluid begins to surround the growing baby. Other developments in the second trimester include:

  • By the end of the second trimester, the baby is about 1 foot long and weighs about 1 pound. If birth occurs at this time, the fetus will attempt to breathe, but survival is unlikely before week 24.
  • The baby’s genitals are fully formed by week 14. The sex of the child can be determined using ultrasound.
  • By weeks 12 to 14, the fetal heartbeat can be heard with a stethoscope.
  • Hearing is well established by 24 weeks, when the baby begins to respond to outside sounds. The baby can now hear the mother’s voice and is likely to recognize it after birth.
  • Beginning at 16 weeks, the baby is sensitive to light, and by 28 weeks a baby can open his or her eyes and turn the head.

Need To Know:

By the fourth month, many women feel the first signs of life in their abdomen. The baby starts to kick and move. The amount of movement varies as the pregnancy continues. Babies move more at night and after the mother eats. As long as the mother feels the baby moving vigorously, it is likely in good health. If the movements decrease day by day, the mother should tell her physician.

The Third Trimester (Weeks 27 to 40)

The third trimester begins at the 27th week of pregnancy and lasts until birth. The baby continues to grow and put on weight throughout the last trimester of pregnancy. During the last month, the fetus grows about one-half pound per week.

By the ninth month, the baby usually settles into a position delivery, with the head down and arms and legs pulled up tightly against the chest.

Other developments during the third trimester include:

  • By week 28, the baby’s eyes are open and a child born at this time can move its limbs and cry weakly. However, the infant will weigh only about two pounds. But because of recent advances in caring for premature babies, 90 percent of babies born at this stage will survive.
  • The baby’s movements become more frequent and vigorous.
  • The baby is considered full-term after 37 weeks from the beginning of the mother’s last period.

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