The fetal heart rate can be seen on ultrasound as early as 5 weeks 1 days of the pregnancy, and usually it's seen by 6 weeks and 0 days.
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 in and by itself the absence of a fetal heart rate at that time does not mean it's a miscarriage. 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 and 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 a approximately 50% miscarriage rate.
|Fetal Heart Rate||Miscarriage Rate|
|Under 90 BPM||86%|