A primi at 20 weeks consulted obstetrician and advised a scan and was showing single live fetus and later she was admitted in another hospital under another Obstetrician with a report of twin pregnancy scan done at 34 wks and has delivered a twin health female babies. What are the chances of missing a twin pregnancy at 20th to 22nd weeks on ultrasound?

Any studies or evidence and possibilities? Kindly give your views.