U/s probs- need some advice please

Am

Hi. I've just had my first scan today at 12 weeks, although they think I'm in fact 13 weeks.... Problem is i was supposed to be having the nuchal fold test but it couldn't be done due to baby being awkward and lying in a difficult place. I've now got to go back in 2 weeks for the quad test and something referred to as HC in my notes. Does anyone have any info on what HC is and the quad test?

Thanks xxx