20 week ultrasound, issues seeing heart and face?

We had our 20 wk ultrasound yesterday and due to baby's lying position and hands over face, we were told they couldn't get a clear view of his heart and facial bones. They confirmed there is a healthy heartbeat and 4 chambers, but could not see the left side. Also, the hands over the face that we found so cute also meant they were unable to rule out the chromosomal abnormalities they check the heart and face for (like Down's syndrome, cleft lip,etc). I will be re-checked in 4 weeks, but am VERY stressed. They said all is normal and it's precautionary, but why can't I find anything about this in the books?