Baby measuring below the 3rd percentile at 30 weeks

I have been getting regular scans since 20 weeks because baby was measuring around the 13th percentile and my doctor said that’s fine but they start to worry when baby drops below the 5th.. I just had my 30 week scan and she has dropped into the 3rd percentile.. I now have to be seen by maternal fetal medicine, was marked for high risk pregnancy and told that I will have to have the baby by or before 37 weeks.. has anyone else ever experienced this or something similar? So far the placenta and everything seems to be working just fine possibly just a very small baby?