How early can a doctor hear the baby's heartbeat?
Before the age of radar and sonar as applied medically, it was the ol' stethoscope that was used most often to listen to the baby. Unfortunately this meant an audience of one—the one who sat between the earpieces of the stethoscope.
With modern obstetrics came the other disciplines of science as applied to medicine, and one of these benefits was ultrasound. Whether you actually see the baby on a screen or hear the baby's heartbeat on a little speaker, the technology is similar:
Physical structures are sensed by reflection of sounds emitted toward them and by changes in motion, and electrical signals are reproduced based on these reflections. Depending on the device used, these electrical "signatures" are translated into either audible sounds or visible images. The visible images are readily seen, but audible signals may take a little longer into the pregnancy to pick up because, unless you can see what you're aiming for with ultrasound pictures, trying to aim for the fetal heart can be a daunting challenge when the baby and heart are small.
With modern Doppler devices, a fetal heartbeat can be heard from 10-12 weeks, although not hearing them up to 12 weeks is not particularly worrisome. With only a stethoscope you'd have to wait until about 18-20 weeks.
Also, the fetal heart rate is much faster in early pregnancy than in later pregnancy. In the first trimester, the fetal heart rate (FHR) is usually above 140/minute, and sometimes as high as 180. By the end of the pregnancy, it usually is around the 120 range. A fetal heart rate of less than a hundred in early pregnancy is not a good sign, so it's important to distinguish it from the maternal rate.
In early pregnancy, picking up the FHR on Doppler may also depend on how well the sound is transmitted via adjacent structures. If the baby is moving around it may be difficult to pick up.
Most pick-ups of the FHR are not in fact the baby's actual heart. Most often it is the swishing of pulsations through the baby's umbilical cord. This counts. The umbilical cord cannot pulsate without the fetal heart pumping blood through it.