Doctor couldn't find hb at 18 weeks...

strawberrytree

I had my 18 week appointment this afternoon, and everything was fine until it was time to listen to the hb. My doctor tried for about 5-10 minutes with the Doppler, both sides, and couldn't find LO's heartbeat. 

I have had a straightforward pregnancy so far, no complications or anything. My doctor didn't seem overly concerned, saying LO was probably just hiding, but she did say she would try and get me an ultrasound appt ASAP instead of 2 weeks from now. Meanwhile, I'm panicking. I want to go to one of those elective u/s places to hear the h/b if we can get an appt this weekend, but in the meantime has this happened to anybody before? With a happy ending, preferably (although I can take sad stories too- I want to prepare myself if there's a good chance something happened).