All baby heart monitors designed for fetal heartbeat monitoring is equipped with either a 2MHz or 3MHz. With a 2MHz probe the beam is narrow and direct which helps it penetrate deeper (3 – 5cm compared to 1 – 2cm) and find the fetal heartbeat earlier than 3MHz dopplers.
The higher the frequency, the shorter in duration is the period and the shorter is the wavelength
The lower the frequency, the longer in duration is the period and the longer is the wavelength
There is a lot of information available stating that the 3MHz fetal doppler probe actually allows the user to find the heartbeat much easier and earlier. We disagree. In our tests and review we found that a 2MHz doppler probe picked up the fetal heart rate at 8 weeks 80% more of the time than a 3Mhz probe. Part of the argument for the 3MHz probe enabling easier detection of the heartbeat comes from the belief that because the higher frequency probe has a wider beam the heartbeat is easier to find. When in fact after 12 weeks gestation either probe is easy to find the heartbeat with. This and the fact that a 2MHz probe is much more useful for overweight woman is why we recommend the 2MHz probe and feel it’s a better all around probe.