Fluid between placenta and uterine wall

Emily
I'm currently 18.4 weeks pregnant i had to have an mri done earlier this week and they found about 3 cm of fluid between my placenta and uterine wall. Mt doctor recommended that i see a high risk specialist he said it could be bleeding that clotted itself or something else. Has any one ever gone through this before or came across this issue? If so should i be worried? Thank you!