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At 4 weeks, the baby is developing:

The nervous system (brain and spinal cord) has begun to form.

The heart begins to form.

Arm and leg buds begin to develop.

The baby is now an embryo and 1/25 of an inch long.

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At 5 weeks, the embryo begins to develop into a fetus:

All major organs have begun to form.

The baby's heart begins to beat.

The arms and legs grow longer.

Fingers and toes have begun to form.

Sex organs begin to form.

The face begins to develop features.

The umbilical cord is clearly visible.

At the end of 8 weeks, the baby is a fetus, and is nearly 1 inch long, weighing less than ⅛ of an ounce.

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The end of the first trimester is at about week 9:

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.

Eyelids close to protect the developing eyes. They will not open again until week 28.

Head growth has slowed, and the baby is about 3 inches long, and weighs almost an ounce.

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The baby continues to develop:

The musculoskeletal system continues to form.

Skin begins to form and is nearly translucent.

Meconium develops in your baby's intestinal tract. This will be your baby's first bowel movement.

The baby begins sucking motions with the mouth (sucking reflex).

The baby is about 4 to 5 inches long and weighs almost 3 ounces.

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Your Growing Baby's Changes Through the Second Trimester | WebMD

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At about 18 weeks in the second trimester, the baby continues to develop:

The baby is more active.

The baby is covered by fine, feathery hair called lanugo and a waxy protective coating called vernix.

Eyebrows, eyelashes, fingernails, and toenails have formed. The baby can even scratch itself.

The baby can hear and swallow.

The baby is about 6 inches long and weighs about 9 ounces.

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By 23 weeks, even more changes occur for the growing baby:

The baby's bone marrow begins to make blood cells.

Taste buds form on your baby's tongue.

Footprints and fingerprints have formed.

Hair begins to grow on your baby's head.

The lungs are formed, but do not yet work.

The baby has a regular sleep cycle.

If the baby is a boy, his testicles begin to descend into the scrotum. If the baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs has formed in the ovaries.

The baby stores fat and weighs about 1½ pounds, and is 12 inches long.

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At 28 weeks in the third trimester, the baby's development continues:

The baby's bones are soft but fully formed.

Movements and kicking increase.

The eyes can open and close.

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 (fine hair) begins to fall off.

The baby is gaining about ½ pound a week, weighs about 4 to 4½ pounds, and is about 15 to 17 inches long.

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At 32 weeks, the baby continues development:

The protective waxy coating (vernix) thickens.

Body fat increases.

The baby is getting bigger and has less space to move around. Movements are less forceful, but you will still feel them.

The baby is about 16 to 19 inches long and weighs about 6 to 6½ pounds.

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Finally, from 36 to 40 weeks the last stages of the baby's development occur:

By the end of 37 weeks, the baby is considered full term.

The baby's organs are capable of functioning on their own.

As you near your due date, the baby may turn into a head-down position for birth.

Average birth weight is between 6 pounds 2 ounces to 9 pounds 2 ounces and average length is 19 to 21 inches long.

Most full-term babies fall within these ranges, but healthy babies come in many different weights and sizes.