According to Babycenter: Your baby now weighs about 2 1/2 pounds (like a butternut squash) and is a tad over 15 inches long from head to heel. His muscles and lungs are continuing to mature, and his head is growing bigger to make room for his developing brain. To meet his increasing nutritional demands, you'll need plenty of protein, vitamins C, folic acid, and iron. And because his bones are soaking up lots of calcium, be sure to drink your milk (or find another good source of calcium, such as cheese, yogurt, or enriched orange juice). This trimester, about 250 milligrams of calcium are deposited in your baby's hardening skeleton each day.
Your baby's very active now. Your practitioner may ask you to spend some time each day counting kicks and will give you specific instructions on how to do this. Let her know if you ever notice a decrease in activity. You may need a nonstress test or biophysical profile to check on your baby's condition.
Some old friends — heartburn and constipation — may take center stage now. The pregnancy hormone progesterone relaxes smooth muscle tissue throughout your body, including your gastrointestinal tract. This relaxation, coupled with the crowding in your abdomen, slows digestion, which in turn can cause gas and heartburn — especially after a big meal — and contribute to constipation as well.