At 16 weeks, the baby's head is now erect, and those tiny arms and legs are developed -- and his fingernails are also formed. His muscles begin to move energetically. In fact, starting this week, your baby’s movements -- like him kicking and punching and sucking his thumb -- can easily be seen via ultrasound. (Soon, you will be able to feel them yourself.)

His skin doesn't yet have color -- it actually looks pretty transparent, and the veins and arteries are clearly visible.

By the middle of this month, the baby reaches another major milestone -- the external sex organs become distinctly male or female. Facial features, too, are becoming more distinct. As the head has grown, the ears have moved to their proper places on the side of the head. Underneath your babie's gums, teeth are forming, even though they won't appear until several months after your baby's birth. The lips are also well developed.

Growth during this period is remarkable. The fetus is now 5 inches long and weights nearly four ounces -- just big enough to cradle in the palm of your hand.

what site do you get these pictures from?

Hey Tim,

I write the weekly updates. I just do a google image search for "16 weeks fetus" or "fetal development."

Usually pregnancy.org has a lot of good pics, too.