At about 5 weeks gestation, your baby's heart begins to beat. At this point, a normal fetal heart rate is about the same heart rate as the mother's: about 80-85 beats per minute (BPM). From this point, it will increase its rate about 3 beats per minute per day during that first month.
This is so exact, that your doctor or midwife can actually use heart rate to help pinpoint the gestational age of your baby via ultrasound. The miscarriage rate for pregnancies in which a mother has heard or seen a heart beat is less than 1%; however, if your doctor notices that your baby's heart beat is off by a week or more it can indicate that a miscarriage is more likely.
By the beginning of the 9th week of pregnancy, the normal fetal heart rate is an average of 175 BPM. At this point it begins a rapid deceleration to the normal fetal heart rate for the middle of the pregnancy of about 120-180 BPM. There is also a slowing of the normal fetal heart rate in the last ten weeks of pregnancy, though the normal fetal heart rate is still about twice the normal adult's resting heart rate.
Your baby's normal fetal heart rate will also vary naturally, just as your heart rate does. Movement, sleeping and other activities can cause a normal variation. Be sure to talk to your midwife or doctor about concerns that you have with your baby's heart rate.
Some moms feel better when they can monitor the baby's heart beat from home. This use of a doppler in the home is not recommended for most mothers. There are other ways to listen to your baby's heart beat.
DuBose TJ, Cunyus JA, & Johnson L; Embryonic Heart Rate and Age. J Diagn Med Sonography 1990; 6:151-157.
Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.