Now a days, with all the technology we have available to us, only about 3% of all twin pregnancies are undetected until birth. It is possible to have an ultrasound and not see the other baby, though rare.
As was the case for my sister and I, we were a "surprise" set of twins at birth for our parents. The MD only detected one heartbeat and only felt one baby while our mom was pregnant because we were positioned on top of one another. Plus Ultrasounds were very new then and rarely used in regular pregnancy evaluations so there were no U/S pictures to see if more then one of us was lurking inside - LOL. We were quite the Birth-day surprise for our mom and dad! And we were born on mother's day as well that year. What a day that was for our mom.
About.com is a great source for all things about all things but I have found it to be a fantastic and informed source as well for Multiple Birth/Pregnancy Facts and information. Check it out too today!