On the third month of pregnancy (12 weeks) the baby’s bones begin to harden. The baby is already moving spontaneously, but the mother usually can’t feel these movements for awhile yet. The baby’s eyes are large and open. Eyelids will form later. External ears have formed. Besides that, a chin, bits of teeth, a nose, and a forehead and the fingers and toes are almost fully formed. His/ her head is the biggest part of their body. It is about 1/2 the size of the rest of the body. While the head is large, the brain’s structure is similar to what it will be at birth. S/he weighs about 14 grams and is approximately 3.54 inches in total length. By the end of this month mother should be able to hear the baby’s heart beat.

External genitalia begin to differentiate, though it’s still very difficult to tell whether the baby is a girl or a boy without genetic screening. The baby moves from being an embryo to the fetal period, now called a fetus. S/he also loses their tail!

Refered from :http://pregnancy.about.com