Week 12: Fetal Development: Systems and Organs

This week marks a turning point for your baby. The herculean task of developing new bodily structures is nearing an end as most of your baby's systems are fully formed. Now comes the maintenance phase, during which your fetus' systems continue to mature for the next 28 weeks and the organs get to work.

Your little fetus weighs as much as half an ounce now, and the crown-to-rump length is about two and a half inches (the size of a large plum — busily working his or her way through the whole produce department). It's hard to believe (especially from the outside, since you're probably barely showing at this point), but your baby has more than doubled in size during the past three weeks. And that's just on the outside — there's plenty going on inside too. The fetal digestive system is beginning to flex its digestive muscle — literally — as it starts practicing contraction movements, a skill your baby will need after birth to push food through the digestive tract. The pituitary gland at the base of the brain has started producing hormones. And the bone marrow is making white blood cells, which will one day help your baby fight infection (including all those germs passed around the snot-nosed playgroup).

If you haven't already had the pleasure, it's likely that at this month's checkup you'll finally hear your baby's heartbeat — a sound that will make your heart race with joy!