“Pulsed Doppler techniques generally involve greater temporal average intensities and powers than M-mode, and hence greater heating potential, due to the high pulse repetition frequencies and consequent high duty factors that are often used. The British Medical Ultrasound Society guidelines state: So why don’t we use Doppler ultrasound to hear the heartbeat in the first 16 weeks of pregnancy? The next thing we are often asked to do is to listen for the baby’s heartbeat, even if they have clearly seen the pulsations. We show you the flickering of the heart beat tiny heart pulsations that are sometimes difficult to see. As a sonographer, we see the relief in the faces of both parents when we show you the gestation sac containing a yolk sac and tiny fetal pole nestling in the uterus. The first ultrasound scan in pregnancy is usually met with a combination of excitement and trepidation. Failure to identify any cardiac activity in a fetal pole whose overall length is greater than 4mm is an ominous sign. In a very early scan at 5-6 weeks just visualising a heart beating is the important thing. This is completely normal in a healthy ongoing pregnancy, and will increase to between 120-180 bpm by 7 weeks. If heart pulsations can be seen, they are usually slow at about 100 bpm (beats per minute). The fetal pole can normally be seen lying on the edge of the yolk sac, and in these very early stages, it may or may not have a heartbeat. This is called the fetal pole or fetal node. Towards the end of the 5th week after the first day of the last period (LMP), and just 3 weeks after conception, a tiny embryo can be seen as a thin white line measuring approximately 2-3mm in length.
Using transvaginal ultrasound, the fetal heartbeat can be seen flickering before the fetal pole is even identified.
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