High amniotic fluid

*cross posted*

I’m 31 weeks with b/g twins and just got a call from the doctor saying that they want to do a follow up ultrasound because twin B had high levels of amniotic fluid at my 29 week scan. They told me this at MFM but said that she had more fluid because she was bigger than twin A so I assumed it was nothing to be concerned about but two weeks later, my OB is telling me differently. She said that it could be caused by a birth defect but my anatomy scan looked good so she doesn’t know. Does anyone have experience with this??