In the first trimester, the baby's heart rate should start somewhere around 100 beats per minute (bpm) around 6 weeks gestation (the time of first detection), peak at 9 weeks (sometimes even reaching levels as high as 180 beats per minute) and then gradually decreases as the fetus approaches term.
When the heart rate is slower than expected, the doctor may note that there's some cause for concern and recommend a followup ultrasound to check the baby's development. A slow heart rate is a cause for concern because studies show higher odds of miscarriage when the baby has a heart rate of less than 100 beats per minute at 6.2 weeks of pregnancy or less than 120 beats per minute at 6.3 to 7 weeks.
If your ultrasound revealed that your baby had a slow heart rate, you are probably scared and concerned, especially if you have been searching for information on what this might mean. You may feel frustrated that you have to wait a week for a followup. But unfortunately there is no way to tell what is happening without that wait. Sometimes the baby's heart rate will normalize, and then the pregnancy will continue without further complications. But sadly sometimes the outcome goes the other way. There is nothing you or your doctor can do to affect the ultimate outcome. When pregnancies miscarry after detecting a slow fetal heart rate, the reason is often chromosomal abnormalities that were present at conception.
Doubilet, P. M. and C. B. Benson. "Embryonic heart rate in the early first trimester: what rate is normal?" Journal of Ultrasound in Medicine 1995. Vol 14, Issue 6 431-434.
Doubilet, P.M., C.B. Benson and J.S. Chow, "Long-term prognosis of pregnancies complicated by slow embryonic heart rates in the early first trimester." Journal of Ultrasound in Medicine 1999. Vol 18, Issue 8 537-54.