They would be able to find the twin with U/S no problem at 30 weeks. Boys CAN have 3 lines in earlier U/S and appear to be girls, so I would say she probably had a boy that wasn't far enough developed and by the next scan it was. If they are not able to find a twin at 26 weeks then either the tech is seriously misqualified or the twin is SO small there is no way it could ever be viable. It has been obvious I have 2 in there from 8 weeks. Now they can barely get a shot of one without getting some part of the other (and we just had our 26 week U/S last week). I'd get the idea of twins out of your head. If they've done U/S and couldn't find one, then there isn't one there.
Do you have to be quite so blunt??
Maybe there is a simple explanation where the other heartbeat is coming from, i'm not a doctor so i dont have a clue. If you read the original post she clearly says that her friend has to go have a specialist scan to double check if there is a hiding twin in there so the idea is not 'in her head' it has been mentioned by someone in the medical profession.
Squeeks, hope your friend gets some answers soon xx