Is having a marginal previa/low-lying placenta considered high risk?

Marginal previa. A marginal previa or a low lying placenta is usually diagnosed by ultrasound. Early in a pregnancy ultrasound can over diagnose the condition. Risk will depend upon whether you have had any bleeding with the diagnosis. A 28 week ultrasound is usually recommended to better visualize where the placenta actually is.
Yes. 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.