I had a private ultrasound today in the UK at 6 weeks and 5 days. The person doing the scan wasn’t a medical professional, and halfway through she became really focused and quiet. When I asked what she was looking at, she said she was double checking for twins because of the yolk sac positioning. In the end, she told me she was pretty certain there was only one baby, but now I’m curious — how likely is it for a twin pregnancy to be missed this early on?