The heart beat normally begins at four weeks but can't be heard with a stethoscope until eight weeks, according to Mayo Clinic online. Vaginal ultrasound can pick up the heart beat at about five-six weeks.
Amazingly enough, a fetus heart forms within the first month of pregnancy, so it beats as soon as it is formed, but cannot be heard by stethescope until 9 weeks.
The babies heart starts beating at about 5 weeks after gestation. You wont be able to hear it with the doppler until about 10 weeks. But if you go in for an ultrasound before the 10th week you should be able to see a little flicker in the fetus which is the heart beating.