Heartbeat on Doppler.

Krystle • Married to my best friend. Baby #2 is due september 23, 2015 and baby #1 will turn 2 on June 19!
I'm 12 weeks today and the midwife couldn't find the heartbeat on the doppler. I'm only 105 pounds so it worries me that she didn't hear anything. She didn't seem concerned though and said my uterus is probably still too far down. Has this happened to anyone else?