When we perform an early pregnancy ultrasound we always measure the embryonic heart rate. The heart rate offers additional important information on the health and future success of the pregnancy.
We have mentioned that sometimes the age of the pregnancy does not match your last menstrual period so we use the crown rump length to determine the age and to calculate the estimated due date and we measure and use the heart rate as an additional vital sign. Studies have proven that the lower limit of normal is 100 beats per minute up to 6.2 weeks gestation and 120 beats per minute at 6.3 to 7.0 weeks.
We only use a motion mode to calculate heart rate in the early weeks because using pulsed wave Doppler may elevate temperatures that can damage biological tissue in human embryos. You will be able to see the heart flutter on the ultrasound but you won’t be able to hear the heart beating.
A slow heart rate in the first trimester is often an indicator of poor pregnancy outcome. At our centre would always offer you an additional scan in 10 to 14 days to confirm viability.
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