My sister is carrying big at 8 weeks, but the 6 week ultrasound detected 1 embryo. Can ultrasounds miss twins?

My sister is carrying big at 8 weeks, but the 6 week ultrasound detected 1 embryo. Can ultrasounds miss twins?
Question by debbierlus: My sister is carrying big at 8 weeks, but the 6 week ultrasound detected 1 embryo. Can ultrasounds miss twins?
There is history of fraternal twins on my father's side. She is thirty nine, not overweight, and it is her FIRST pregnancy, so it isn't because she is on her second. Anyone have an early ultra-sound with one baby showing, and then find out later

6 week ultrasound