How will my baby develop week by week?

At 4 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 1/25 of an inch long.

At 8 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 8 weeks, your baby is a fetus and looks more like a human. Your 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. 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.

Source: http://www.medicinenet.com/pregnancy/page4.htm#develop