The fetal heart activity is one of the earliest signs that the pregnancy is viable. It can be seen by transvaginal sonogram (TVS) as early as 22 days after fertilization/ovulation or 5 weeks 1 day after the last menstrual period. However, many times it's not seen until exactly 6 weeks.

By the time the embryo measures 2 mm, the fetal heart rate activity should be seen and normal embryos start at under 85 beats per minute heart rate. In 5-10% of normally progressing pregnancies when the embryos measure between 2 and 4 mm, the fetal heart rate cannot be seen, so the absence of a fetal heart rate at that time does not mean it's automatically a miscarriage. However, by the time the embryo measures 5 mm, all embryos should have a heart rate.

Between 5 to 9 weeks of gestation, there is a rapid increase in the mean heart rate. Normal embryos start at under 85 beats per minute heart rate. It then increases from 110 to 175 beats per minutes (bpm) and stabilizes around 120-160 beats per minute. 

The lower the fetal heart rate is around 6-8 weeks, the higher the miscarriage rate can be predicted. A fetal heart rate below 70 beats per minute around 6-8 weeks usually predicts a miscarriage. A fetal heart rate below 90 beats per minutes is associated with a 86% miscarriage rate, and a fetal heart rate below 120 bpm is associated with an approximately 50% miscarriage rate.

Fetal Heart Rate and Risk of Miscarriage
Fetal Heart Rate Miscarriage Rate
<70 100%
70-79 91%
80-90 79%
Under 90 BPM 86%

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