We told lady doing so an that we didn't want to find out the sex of our twins. However, at no point did she warn us to look away and now I'm 99.9999999%sure one was a boy. A bit miffed but then I'm wondering if she didn't bother saying as it's too hard to guess to an untrained eye? We have two boys and found out the sex of the second one at the 20 week scan and now I'll be stunned if they weren't boy bits I saw (second twin was lying funny, she struggled to get any measurements so didn't see anything-phew)!