First Trimester (week 1 – week 12)

At  4 weeks:

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

At 8 weeks:

  • All major organs and external body structures have begun to form.
  • The 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 8 weeks, the baby is a fetus and looks more like a human. The baby is nearly 1 inch long and weighs less than 1/8 of an ounce.

At 12 weeks:

  • The nerves and muscles begin to work together. The baby can make a fist.
  • Eyelids close to protect the developing eyes. They will not open again until the 28th week.
  • Head growth has slowed, and the baby is much longer. Now, at about 3 inches long, the baby weighs almost an ounce.

Second Trimester (week 13-week 28)

At 16 weeks:

  • The external sex organs show if the 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.
  • Muscle tissue and bone continue to form, creating a more complete skeleton.
  • Skin begins to form. It is almost transparent.
  • Meconium (mih-KOH-nee-uhm) develops in the baby's intestinal tract. This will be the baby's first bowel movement.
  • The baby makes sucking motions with the mouth (sucking reflex).
  • The baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.

At 20 weeks:

  • The baby is more active. There might feel slight fluttering.
  • The baby is covered by fine, downy hair called lanugo (luh-NOO-goh) and a waxy coating called vernix. This protects the forming skin underneath.
  • Eyebrows, eyelashes, fingernails, and toenails have formed. The baby can even scratch itself.
  • The baby can hear and swallow.
  • Now halfway through the pregnancy, the 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 the baby's tongue.
  • Footprints and fingerprints have formed.
  • Real hair begins to grow on the baby's head.
  • The lungs are formed, but do not work.
  • The hand and startle reflex develop.
  • The baby sleeps and wakes regularly.
  • If the baby is a boy, his testicles begin to move from the abdomen into the scrotum. If the baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs have formed in the ovaries.
  • The baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, the baby weighs about 1½ pounds.

Third Trimester (week 29-week 40)

At 32 weeks:

  • The baby's bones are fully formed, but still soft.
  • The baby's kicks and jabs are forceful.
  • The eyes can open and close and sense changes in light.
  • Lungs are not fully formed, but practice "breathing" movements occur.
  • The baby's body begins to store vital minerals, such as iron and calcium.
  • Lanugo begins to fall off.
  • The baby is gaining weight quickly, about one-half pound a week. Now, the baby is about 15 to 17 inches long and weighs about 3 to 334pounds

At 36 weeks:

  • The protective waxy coating called vernix gets thicker.
  • Body fat increases. The baby is getting bigger and bigger and has less space to move around. Movements are less forceful, but the mother may report  feeling the baby stretch and wiggle.
  • The baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.

At 37-40 weeks:

  • By the end of 37 weeks, the baby is considered full term. The baby's organs are ready to function on their own.
  • Towards the end of the due date, the baby may turn into a head-down position for birth. Most babies "present" head down.
  • At birth, the 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.