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Your Baby’s Development in the Womb

Pregnant woman with ultrasonic baby shot isolated on white

The First Trimester

Pregnancy begins when a sperm fertilizes an egg. Because we often don’t know exactly when conception takes place, doctors calculate the timetable for your pregnancy based on the first day of your last menstrual cycle. Your due date is estimated to be at 40 weeks.
Dramatic change occurs in the embryo during the 5th to the 10th week of pregnancy. Here’s what happens during this stage:

  • All of the baby’s major organs are developed
    o Nervous system
    o Heart
    o Face
    o Arms and legs
    o Sexual organs
    o Muscles and movement
    o Body parts begin to develop
  • The embryo’s cells begin to multiply and change into hundreds of different types of cells needed to make up the human body
  • The placenta develops which provides oxygen, water and air to the embryo through the umbilical cord
  • The amniotic sac develops which surrounds and protects the developing baby

The final two weeks of the first trimester are referred to as the fetal state. During this period of time the fetus gains weigh, and organs and body parts continue to develop. By the end of the 12th week, the fetus is usually about 3 inches in length.

The Second Trimester

The second trimester occurs during week 13 through 26 of your pregnancy. This is a period of continued growth and development for the baby. During this period, the baby’s head is the biggest part of its body, but the rest of the body will get longer and by the end of the second trimester your baby may be 9 inches or longer.
Around the 20th week you may begin to feel your baby move. These movements help develop muscles as your baby kicks, such and opens and closes its hands. A few weeks into the second trimester, an ultrasound may be able to confirm the baby’s sex. Other highlights include:

  • The baby begins to hear sounds
  • The baby’s eyes may open around the 20th week
  • The baby develops fingerprints and footprints
  • Fine hair begins to appear

The Third Trimester

The third trimester runs from the week 27 to the end of the pregnancy. At this point the baby’s organs and body parts are developed and working. The third trimester is a time for everything to continue to grow and mature. The lungs are the last major organ to complete their development. Other highlights from the third trimester include:

  • Kicks, punches and moving often occur in the first weeks of this trimester
  • As the baby gets larger with less room in the uterus, there may be fewer kicks, replaced by stretches and rolls
  • Eyelashes and eyebrows develop and sometimes a full head of hair
  • Most babies move to a head-down position near the end of the trimester in preparation for birth
  • The majority of babies weigh between 6 and 9 pounds at birth and measure 19 to 21 inches long

And, did you know that only 5% of babies are born on their actual “due date”?

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