Using transvaginal ultrasound, the fetal heartbeat can be seen flickering before the fetal pole is even identified. 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. This is called the fetal pole or fetal node. 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. If heart pulsations can be seen, they are usually slow at about 100 bpm (beats per minute). This is completely normal in a healthy ongoing pregnancy, and will increase to between 120-180 bpm by 7 weeks. In a very early scan at 5-6 weeks just visualising a heart beating is the important thing. Failure to identify any cardiac activity in a fetal pole whose overall length is greater than 4mm is an ominous sign.
The first ultrasound scan in pregnancy is usually met with a combination of excitement and trepidation. 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. We show you the flickering of the heart beat; tiny heart pulsations that are sometimes difficult to see. 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.
So why don’t we use Doppler ultrasound to hear the heartbeat in the first 16 weeks of pregnancy?
The British Medical Ultrasound Society guidelines state:
“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. In the case of spectral pulsed Doppler, the fact that the beam is held in a fixed position during an observation leads to a further increase in temporal average intensity. Colour flow mapping and Doppler power mapping involve some beam scanning, and so generally have a heating potential that is intermediate between that of B- or M-mode and that of spectral pulsed Doppler”.
Cardiac activity should therefore only be demonstrated by a 2-dimensional video clip or M-mode imaging. The M-mode ultrasound collects information along the length of the beam. This disperses energy along the way and reduces the risk for heat cavitation in the fetus. The use of pulsed Doppler in the first trimester is unsafe and should be discouraged, and it’s use does not outweigh the risk. The purpose is proof – proof of cardiac activity. We can do that safely with M-mode and obtain a heart rate.
So, this is the reason that pulsed wave Doppler should never be used in the first trimester. As sonographers, we are doing everything we can to protect your baby from unnecessary risk and are happy to show you the beating heart from as early as 6 weeks gestation. Some operators offer to record your baby’s heart in these early stages, but this is unsafe practice and should not be permitted.
Ultrasound equipment should only be used by people who are fully trained in its safe and proper operation; operators must have an appreciation of the potential thermal and mechanical bio-effects of ultrasound and demonstrate a full awareness of equipment settings. They have a good understanding of the effects of machine settings on power levels. Doppler (which is used to look at blood flow) is never used in early pregnancy when your baby is still developing, particularly with a transvaginal probe. However a transvaginal scan is perfectly safe for you and your baby and this is the standard procedure for an early scan of less than 8 weeks.