you should be able to hear it within a couple weeks through a stethascope, but once again, if baby is really far back or hidden behind the placenta, it may take a while to locate.
How far along are you? If doc is using a Doppler ive heard more experienced mommy's say that it is annoying to the fetus and they move away from it making it hard or impossible to hear a heart beat.
how far along are you? A couple things can make it hard. If you're still early, baby sits very low in the pelvis and it's hard to get a dopplar to pick up anything (that's how it was with my first attempt), and later if baby moves around a lot, or if it likes to hide farther to the back, they might have trouble locating the heartbeat to hear it. My second time trying to hear it we caught it really briefly and then baby scuttled away and we couldn't get it again. As baby gets bigger and has less room to run away, it will be easier to locate the heartbeat. As long as you know it's there from the ultrasound, baby will be fine, so don't worry!
WEll how far along are u ?
Your placenta could be in the way.. That's all I can think of.