This week marks a turning point for your baby. At 12 weeks pregnant,the herculean task of developing new bodily structures is nearing anend as most of your baby's systems are fully formed. Now comes themaintenance phase, during which your fetus' systems continue to maturefor the next 28 weeks and the organs get to work.
 
Yourlittle fetus weighs as much as half an ounce now, and the crown-to-rumplength 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'reprobably barely showing at this point), but your baby has more thandoubled in size during the past three weeks.  And that's just on theoutside — there's plenty going on inside too. The fetal digestivesystem is beginning to flex its digestive muscle — literally — as itstarts practicing contraction movements, a skill your baby will needafter birth to push food through the digestive tract. The pituitarygland at the base of the brain has started producing hormones. And thebone marrow is making white blood cells, which will one day help yourbaby fight infection (including all those germs passed around thesnot-nosed playgroup).  

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* I got this from WhatToExpect.com