A lady I cared for during labour twins were not diagnosed until 22weeks. At her morphology scan it was picked up the baby had a cardiac anomaly and the mother was transferred to Sydney for a review with the experts in the field. Turns out there were 2 in there and some how the sonography was picking up both of the hearts at the same time which made it look like the 1 baby's heart was not normal. Two healthy ID twins we're born some months later (they did share the same sac) the woman had 2 prior scans and only 1 baby was seen. She was also extremely thin so it had nothing to do with her body habitus just the way the babies were lying.