A patient with 20 weeks gestation consulted an Obstetrician and she has advised a US Scan and revealed a single alive fetus and there were no further visits to the doctor. later she was admitted to another hospital with a US scan report evidenced with twin pregnancy done at 34 weeks gestation. She delivered a twin healthy and alive female babies.

What are the chances of missing a twin pregnancy at 20 weeks of gestation and evidence based possibilities in US Scan?