Doctor says placenta is previa now at 16 weeks but might migrate. how can it move like that?

It doesn't: As the uterus grows, it pulls the placental implantation with it. It's like placing an ink dot on a deflated balloon...Blow it up and the dot "moves" higher, stretched out of the way. A placenta previa is more common earlier on and usually not a biggie unless it's still blocking the cervical outlet after 24 weeks. Make sure you get checked after 24 weeks and tell your doc about any bleeding.

Uterine stretching: Previa: Placenta overlies the cervix/birth canal. Gr 1 means placenta is close, Gr 4 means it's directly overlying/blocking the baby's exit -- high chance of preterm labor & bleeding; you'd need a C/S at 36-37wks. More common in woman w/ hx of C/S & smoking. Not much you can do but wait. If dx'ed early, the uterus still grows/stretches and a placenta can migrate away even in 3rd tri. Follow w/ US.