Fetal Heart Beat

 

 


Fetal Doppler

Fetal heart at 16 weeks. There is a distinctly mechanical element to the flow.
Placenta and fetal movement at 16 weeks. This is more uniform, smooth flow.
Placenta and mother's iliac flow at 16 weeks. The mother's pulse is much slower than the fetal pulse.
Fetal heart at 28 week. There is more complexity to the heartbeat and the signal is much stronger.
Placenta at 28 weeks. Placental flow continues to be smooth and not turbulent.
Maternal iliac vessels at 28 weeks. The pulse is slow, relative to the fetus, and high-pitched, reflecting the rapid flow through the vessels.

 

Although the fetal heart begins beating as early as the 5th week after the LMP, your ability to detect it will be limited by your equipment.

An ultrasound machine usually will enable you to see a heartbeat by 5 to 6 weeks gestation if equipped with a vaginal probe. Abdominal ultrasound will usually see the heartbeat by the 7th-8th week of pregnancy.

If you use a Doppler ultrasound fetal heartbeat detector, you can, with effort, usually hear the heartbeat by 12-14 weeks gestation and routinely after that.

  • You aren't really hearing the heartbeat. You are hearing the amplified "beat frequency" generated by the interaction between the outgoing ultrasound signal, and the returning ultrasound signal. When the outgoing signal is reflected back by a moving object (fetal heart), then the returning signal has a slightly higher frequency (if the object is moving toward the transducer), or slightly lower frequency (if the object is moving away). This is called the Doppler shift. Every so often, the peaks and valleys of these slightly different frequencies are superimposed on each other, creating a much louder sound, that happens to be in the audible range. It is this sound that you are hearing.

The normal rate is generally considered to be between 120 and 160 beats per minute.

  • The rates are typically higher (140-160) in early pregnancy, and lower (120-140) toward the end of pregnancy.

  • Past term, some normal fetal heart rates fall to 110 BPM.

  • There is no correlation between heart rate and the gender of the fetus.

Use a coupling agent (eg, Ultrasound jel, surgical lubricant, or even water) to make a good acoustical connection between the transducer and the skin.

Doppler fetal heartbeat detectors are moderately directional, so unless you happen to aim it directly at the fetal heart initially, you will need to move it or angle it to find the heartbeat.

Confirm a normal rate, and listen for any abnormalities in the rhythm of the fetal heart beat.

Using a DeLee stethoscope (equipped with a head-mount), you can sometimes hear the heartbeat by 16 weeks but unless you are practiced with it, you won't hear it until 20 weeks, at which time the mother can usually tell you that she feels the baby moving.

Using a conventional stethoscope, you may never hear the fetal heartbeat.


OB-GYN 101: Introductory Obstetrics & Gynecology
2003, 2004, 2005 Medical Education Division, Brookside Associates, Ltd.
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