I didn't have a clue, and if I hadn't been lying down when they told us I'd have fallen over with shock! My DH looked like this
for about two days... Found out 3 weeks ago.
And it was only after I found out people have mentioned twin insurance! It's about £45 and pays out about 1k I believe. Not that we'd have taken it out anyway, the midwife only found one heartbeat the week before and I had no reason to suspect twins - although my uncles are, there's no other family history and it never crossed my mind.
Takes some getting your head around ime!