Our fetus is about three inches long and the size of a peach at 13 weeks old. Starting about now, babies begin growing at different paces, some faster than others, some more slowly, though they all follow the same developmental path. Growing at a universally breakneck speed now is our baby's body as it tries to catch up to the head in terms of size. Though our baby's head is about half the size of its body now, by the time the baby is ready to make his or her entrance into the world, the head will be only one-fourth as large as the body.
The intestines are also in for some big changes right now. Up till this point, they've been growing in a cavity inside the umbilical cord; but now they're moving to their permanent (and more conveniently located) address, in the abdomen. And to serve our growing baby's needs, the placenta is also growing. It weighs about an ounce now and will weigh one to two pounds at birth (something else you'll soon be able to blame your weight gain on!).
Also developing this week: baby's vocal chords. Because sound can't travel through your uterus (our baby's current habitat), we won't be able to hear any sounds or cries just yet, but oh boy (or girl) — those vocal chords will get a good workout once that baby is born.