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Your Developing Baby

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First trimester (week 1-week 12)

At four to five weeks:

  • Your baby's brain and spinal cord have begun to form.
  • The heart begins to form.
  • Arm and leg buds appear.
  • Your baby is now an embryo and one-twenty-fifth inch long.

At eight weeks:

  • All major organs and external body structures have begun to form.
  • Your baby's heart beats with a regular rhythm.
  • The arms and legs grow longer, and fingers and toes have begun to form.
  • The sex organs begin to form.
  • The eyes have moved forward on the face and eyelids have formed.
  • The umbilical cord is clearly visible.
  • At the end of eight weeks, your baby is a fetus and looks more like a human. Your baby is nearly 1 inch long and weighs less than one-eighth ounce.

At 12 weeks:

  • The nerves and muscles begin to work together. Your baby can make a fist.
  • The external sex organs show if your baby is a boy or girl. A woman who has an ultrasound in the second trimester or later might be able to find out the baby's sex.
  • Eyelids close to protect the developing eyes. They will not open again until the 28th week.
  • Head growth has slowed, and your baby is much longer. Now, at about 3 inches long, your baby weighs almost an ounce.
Learn more: First Trimester (week 1-12)

    Second trimester (week 13-week 28)

    At 16 weeks:

    • Muscle tissue and bone continue to form, creating a more complete skeleton.
    • Skin begins to form. You can nearly see through it.
    • Meconium (mih-KOH-nee-uhm) develops in your baby's intestinal tract. This will be your baby's first bowel movement.
    • Your baby makes sucking motions with the mouth (sucking reflex).
    • Your baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.

    At 20 weeks:

    • Your baby is more active. You might feel slight fluttering.
    • Your baby is covered by fine, downy hair called lanugo and a waxy coating called vernix. This protects the forming skin underneath.
    • Eyebrows, eyelashes, fingernails, and toenails have formed. Your baby can even scratch itself.
    • Your baby can hear and swallow.
    • Now halfway through your pregnancy, your baby is about 6 inches long and weighs about 9 ounces.

    At 24 weeks:

    • Bone marrow begins to make blood cells.
    • Taste buds form on your baby's tongue.
    • Footprints and fingerprints have formed.
    • Real hair begins to grow on your baby's head.
    • The lungs are formed, but do not work.
    • The hand and startle reflex develop.
    • Your baby sleeps and wakes regularly.
    • If your baby is a boy, his testicles begin to move from the abdomen into the scrotum. If your baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs have formed in the ovaries.
    • Your baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, your baby weighs about 1½ pounds.

    Learn more: Second Trimester (week 13-28)

    Third trimester (week 29-week 40)

    At 32 weeks:

    • Your baby's bones are fully formed, but still soft.
    • Your baby's kicks and jabs are forceful.
    • The eyes can open and close and sense changes in light.
    • Lungs are not fully formed, but practise "breathing" movements occur.
    • Your baby's body begins to store vital minerals, such as iron and calcium.
    • Lanugo begins to fall off.
    • Your baby is gaining weight quickly, about one-half pound a week. Now, your baby is about 15 to 17 inches long and weighs about 4 to 4½ pounds.

    At 36 weeks:

    • The protective waxy coating called vernix gets thicker.
    • Body fat increases. Your baby is getting bigger and bigger and has less space to move around. Movements are less forceful, but you will feel stretches and wiggles.
    • Your baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.

    Weeks 37–40:

    • At 39 weeks, your baby is considered full-term. Your baby's organs are ready to function on their own.
    • As you near your due date, your baby may turn into a head-down position for birth. Most babies "present" head down.
    • At birth, your baby may weigh somewhere between 6 pounds 2 ounces and 9 pounds 2 ounces and be 19 to 21 inches long. Most full-term babies fall within these ranges. But healthy babies come in many different sizes.

    Learn more: Third Trimester (week 29-40)

    by Phuong Le