Placental Problem? Please help

Hi - I am 21 weeks pregnant with twins.  I had the AFP test, came back
high for Down Syndrome, had level 2 ultrasound and amnio, all came
back normal.

My perinatologist thinks that the amnio showed that there were no
chromosonal problems but a high Maternal Serum AFP result can mean
there are problems with the placentas.  Such as it stops working or
becomes detatched or I can go into premature labor.

I have been having off an on bleeding (which may be the result of
placental problem or SCH or both); I have very low amniotic fluid in
both sacs (about 2.2 cm in the biggest pocket) and both twins are
lagging behind in growth (they were 16.1 when they should have been 18
weeks).  They are growing though.

Has anyone heard of this or had this happen to them??  Please let me
know.  Thanks for reading.

Gail