I was 14 weeks when I bought mine and found my twins right away. It's a sonoline b from babydoppler.com

Sonoline b I found mine at 9 weeks 6 days burin also really skinny and worked out a lot so no belly chub... I think the lack of extra tissue between the device and baby make it easier to find.

Sonoline B. I got mine at 11 weeks exactly, it took about 15 minutes but I found it, after a few days and getting the hang of it I could find the heartbeat in just a minute.