20 week Ultrasound - stomach full of amniotic fluid

I'm 20 weeks pregnant and recently have had 2 ultrasounds. During the
first US, the radiologist noticed that the baby's stomach was full and
asked me to come back to check it in a week. I went back a week later
and the stomach was still full of amniotic fluid. She booked me in
for another US next week and I'm very nervous - she said to check out
duodenal atresia as this is what she was concerned was the case. She
also said this is a warning sign of Down's Syndrome, but also told me
not to worry - right! That's about all the info I have at this time
and hubbie and I are very concerned.
Has anyone heard of this and is the stomach being full normal? What
else could be the reason for the stomach being full? What other signs
should be present in a US to note Down's?
The first US technician said the Radiologist was being overcautious as
they see the stomach full all the time, but that was at the first US.

Any info is greatly appreciated.
Sincerely,
Jules