16 weeks

Maybe you will be able to start telling a difference from week 13 to week 16 now. Or maybe not.

Here is what BabyCenter.com says for this week: "Get ready for a growth spurt. In the next few weeks, your baby will double his weight and add inches to his length . Right now, he's about the size of an avocado: 4 1/2 inches long (head to rump) and 3 1/2 ounces. His legs are much more developed, his head is more erect than it has been, and his eyes have moved closer to the front of his head. His ears are close to their final position, too. Your baby's heart pumps 25 quarts of blood a day."