Your Developing Baby

First trimester (week 1-week 12)

At four weeks:

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

Baby Development 4 weeks

At eight weeks:

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


At 12 weeks:

  • The nerves and muscles begin to work Your baby can make a fist.
  • The external sex organs show if your baby is a boy or 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 They will not open again until the 28th week.
  • Head growth has slowed, and your baby is much Now, at about 3 inches long, your baby weighs almost an ounce.

Second trimester (week 13-week 28)

At 16 weeks:

  • Muscle tissue and bone continue to form, creating a more complete
  • Skin begins to You can nearly see through it.
  • Meconium (mih-KOH-nee-uhm) develops in your baby’s intestinal 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


At 24 weeks:

  • Bone marrow begins to make blood
  • Taste buds form on your baby’s
  • Footprints and fingerprints have
  • Real hair begins to grow on your baby’s
  • The lungs are formed, but do not
  • The hand and startle reflex
  • Your baby sleeps and wakes
  • If your baby is a boy, his testicles begin to move from the abdomen into the 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 Now at about 12 inches long, your baby weighs about 1½ pounds.

Baby-Developmet-24-36 weeks

Third trimester (week 29-week 40)

At 32 weeks:

  • Your baby’s bones are fully formed, but still
  • Your baby’s kicks and jabs are
  • The eyes can open and close and sense changes in
  • Lungs are not fully formed, but practice “breathing” movements
  • Your baby’s body begins to store vital minerals, such as iron and
  • Lanugo begins to fall
  • Your baby is gaining weight quickly, about one-half pound a 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
  • Body fat 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

Weeks 37-40:

  • By the end of 37 weeks, your baby is considered full 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 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 Most full-term babies fall within these ranges. But healthy babies come in many different sizes.