Yes and no. The ones you buy from Babies R Us, the "heartbeat listeners," are only good in the third trimester.
DOPPLERS, however, which as far as I know aren't easily found in brick and mortar shops, can find the heartbeat much earlier. I'm plus-sized, and found the heartbeat at about 13 weeks. They're considered medical equipment, and the ones the doctor has in the office are of better quality and worth more...which is why, at 11 weeks, the doctor told me she is going to give a listen but will probably not find it.....and lo and behold, it was found within a few seconds.
Just don't confuse the heartbeat listeners with true dopplers. The heartbeat listeners usually state right on the box that they're for use in the last 8 weeks or so of pregnancy.
Good luck!