Angie, sorry I missed this! Yes, it is.
As I understand it, notching is when after each beat of the maternal heart, the bloodflow drops below the average flow when the heart isn't beating, and then pops back up. Here's some pretty pictures: http://www.centrus.com.br/DiplomaFMF/Se ... ga_01.html
This means that the placenta isn't as deeply implanted, because the flow isn't as steady -- it pulses, and not just *up*, with each maternal heartbeat, but *down* a little bit, which is the notch. And it's because the spiral arteries aren't as "remodeled", as they put it -- the placental cells haven't replaced as much of the artery, so the placenta's shallowly implanted.
I've posted a bunch of recent studies in News where they've looked at this notching when it happens in pregnancies following preeclamptic pregnancies, and it's only thought to be *an* indicator, and not a terribly useful one unless it's coupled to other information, because many women with notching go on to have perfectly normal pregnancies. Absent or reverse diastolic flow are solid indicators for delivery, but notching is very common.
Here are some links to those studies: