Can a pregnancy ultrasound miss a twin? I had an early (8 weeks) ultrasound and the doctor said she couldn't rule out twins at this point. Could another baby really be "hiding"? Only one heartbeat detected, so I don't know what to expect.

Possible twins. The fetus is "really small" at 8 weeks, and another could be hiding. Usually they won't say something like that unless there is some hint at twins though, unless it's really a thing they always add to protect themselves. I'd ask why they said that. If it's standard language, don't worry about it. Otherwise, there might be something they were looking at.
This . This can occasionally happen, but it is a very rare occurrence. The experience of the imager is also an important factor to consider. If the examination was performed by an inexperienced ultrasound technologist, they may have been confused by some aspect of the anatomy and questioned the possibility of a twin pregnancy. There are a limited number of places that a baby could "hide" within the uterus; therefore, an experienced ultrasonographer is very unlikely to miss a multigestation pregnancy with two or more fetuses.