I had the same problem when I was about 13 weeks. I freaked out because I couldn't find the heartbeat and I had no problem finding it before. It is just because the heartbeat isn't very strong at 13 weeks. Just make sure to relax and try lying completly flat then try....GOOD LCUK
Are you using a gadget you bought yourself??
If so, I have one too.....and They can sometimes be peices of ****. I usually hear a light heartbeat, but sometimes I can't find it. The baby's fine. It's probably just the doppler thing.
Keep playing around with it....... You never know....... The baby as you know has lot's more room at these stages, then he or she will in the week's to come and it probably just hiding.... I alway's had that problem when I went in........ My babies were alway's laying the wrong way, or just being stubbourn........ And if you still don't find it, stop stressing ........ That doesn't mean anything is wrong...... baby is just enjoying what little time he/she has in all that space! LOL
the baby should be right at your pubic line... right above where your pubic hair stops growing thats where mine was at 13 weeks