Around week 12, several more organs develop enough to begin functioning. The thyroid gland, which helps regulate metabolism, starts working. Two glands important in digestion get busy: the pancreas begins to secrete digestive enzymes, and the gallbladder starts to secrete bile into the digestive tract. The fetus is now about 2.5" (6 cm) in length, the size of a lime, and weighs about .5 oz (14 g). The liver accounts for about 10% of that weight. Together with the spleen, the liver is busy producing blood cells for the fetus.

As the nervous system integrates with the fetus's muscular system, the muscles respond to signals from the brain and the fetus begins to move--kicking, curling her toes, making fists, bending and stretching. She makes faces, too: she frowns, squints, purses her lips, and wrinkles her forehead. As yet, though, these movements lack purpose; there's no specific correlation between stimulus and response.

This is an important point in your pregnancy for you, too. Using ultrasound, your doctor can let you hear the fetal heartbeat this week. It's fast--about 160 beats per minute!