Placenta previa is a condition where the placenta grows over the cervix or is too close to the cervix. This can lead to catastrophic bleeding for both mother and fetus and requires a c-section for safe delivery.
Placenta previa is often suspected early on in pregnancy when an ultrasound suggests the placenta is very close to the cervix, however, weeks later when the ultrasound is repeated the placenta has moved. This is obviously good news, but how does that physically happen? After all, the placenta has invaded into the uterine wall so it can’t exactly pick up its things and walk up to another spot in the uterus like you would on the beach when you realize your planted your umbrella and towel below the high water mark and have now noticed the tide is coming it!
The answer is the placenta doesn’t move, the lower uterine segment beneath it grows. I made a short video to explain:
Dr. Jen Gunter 
Interesting. I’d never thought about it
Reminds me of this birthmark I have. Back when I was a preschooler, it peeked below my shorts (I hated it and would scratch at it, so i remember). Now it only shows when I’m wearing a bathing suit. My knees are a lot further from my hips than they used to be, but the birthmark isn’t much further in inches from my hip.
I am an old lady, and thought I was fairly knowledgeable about my anatomy, and what happens during pregnancy, but I didn’t know this. Thank you. It’s always good to learn a new thing.
We used to call this “placental migration,” an obvious misnomer. Good explanation by Dr. JG.