They can tell as early as 5-6 weeks. I've had two friends, one who found out at 6 weeks and one who found out at 10 weeks that they were carrying twins. I didn't find out I was having twins until I was 5 months along, that was when I had my first ultrasound. Hearing the heartbeat definitely doesn't tell you if you are having twins, as the babies are in different positions and usually doctors aren't looking for a second heartbeat unless you have a change of having multiples(i.e. family history, fertility drugs, etc.). So, it definitely was a surprise for us, but a super awesome one!
at 11:15 PM on Sep. 25, 2010