Is it possible for a scan to reveal only one baby when there are twins?

Yes: Only very early in the pregnancy, like between 5 and 7 weeks. By the time the pregnancy is 8 weeks and beyond, it would be unlikely that an ultrasound would not detect twins.

Yes: In general an experienced sonographer can detect two sacs or two fetuses in the first trimester and beyond. Sometimes one twin will be missed if it is not developing normally (eg, stuck twin) or is in one corner (horn) of an abnormally shaped uterus or if the woman is very large. Those would be very rare exceptions at most centers today.

Yes: A careful us should be able to detect twins whenever they are there but they can still be missed sometimes. In later pregnancy, with an abnormal uterus or in an obese woman, it may be easier to miss the second twin. A quick us just to look at the heart beat could easily miss a second baby. In vanishing twin syndrome, a woman losses 1 twin early in pregnancy, but the other does fine.