Fetal heart rate

A normal fetal heart rate is between 110 to 160 beats per minute (bpm) in the in utero period ¹. Fetal heart rate can vary by 5 to 25 beats per minute. The fetal heart rate may change as your baby responds to conditions in your uterus. An abnormal fetal heart rate may mean that your baby is not getting enough oxygen or that there are other problems.

Fetal heart rate is measurable sonographically from around 6 weeks and the normal range varies during gestation, increasing to around 170 bpm at 10 weeks and decreasing from then to around 130 bpm at term.

Fetal heart beat can be detected as early as 34 days (just under 6 weeks) gestation on good quality, high frequency transvaginal ultrasound, as a crown rump length (CRL) of as little as 1-2 mm.

If a fetal heartbeat cannot be identified with a crown rump length (CRL) ≥7 mm using transvaginal scanning, then embryonal demise can be diagnosed ². Demise can be confirmed with repeated scanning and serial quantitative beta-HCG (human chorionic gonadotropin).

Although the heart muscle (myocardium) begins to contract rhythmically by 3 weeks after conception (from spontaneously depolarizing myocardial pacemaker cells in the embryonic heart) it is first visible on sonography around 6 weeks of gestation ³. While the exact timing of onset of the atrioventricular (AV) electromechanical relationship remains speculation in humans, by 6 weeks post‐conception AV synchrony can be demonstrated using standard Doppler techniques. By 5–6 weeks the normal mean fetal heart rate is 110 beats/min (bpm) ³. With further growth and maturation of the conduction system, including definition of the sinoatrial node as the primary cardiac pacemaker with its highest intrinsic rate of spontaneous depolarization, there is a subsequent increase in the rate to 170 bpm by 9–10 weeks ³. The rise in heart rate is followed by a decrease to 150 bpm by 14 weeks, likely as a consequence of increasing parasympathetic control and improved myocardial contractility. By 20 weeks the average fetal heart rate is 140 (20) bpm with a gradual decrease to 130 (20) bpm by term. In the healthy fetus the heart rate is regular, usually remains between 110 and 180 bpm, and has a beat‐to‐beat variation of 5–15 bpm ³.

Fetal heart rate then increases progressively over the subsequent 2-3 weeks becoming ³:

• ~110 bpm (mean) by 5-6 weeks
• ~170 bpm by 9-10 weeks

This is followed by a decrease in fetal heart rate becoming on average:

• ~150 bpm by 14 weeks
• ~140 bpm by 20 weeks
• ~130 bpm by term

Although in the healthy fetus the heart rate is usually regular, a beat-to-beat variation of approximately 5 to 15 beats per minute can be allowed.

Fetal heartbeat monitoring

Fetal heart rate monitoring measures the heart rate and rhythm of your baby (fetus). Fetal heart rate monitoring is the process of checking the condition of your fetus during labor and delivery by monitoring your fetus’s heart rate with special equipment. Your healthcare provider may do fetal heart monitoring during late pregnancy and labor. Fetal heart rate monitoring lets your healthcare provider see how your baby is doing and may help detect changes in the normal heart rate pattern during labor. If certain changes are detected, steps can be taken to help treat the underlying problem. Fetal heart rate monitoring also can help prevent treatments that are not needed. A normal fetal heart rate can reassure both you and your obstetrician–gynecologist (ob-gyn) or other health care professional that it is safe to continue labor if no other problems are present.

Types of fetal heart rate monitoring

There are two methods of fetal heart rate monitoring in labor. Auscultation is a method of periodically listening to the fetal heartbeat. Electronic fetal monitoring is a procedure in which instruments are used to continuously record the heartbeat of the fetus and the contractions of the woman’s uterus during labor. The method that is used depends on the policy of your ob-gyn or hospital, your risk of problems, and how your labor is going. If you do not have any complications or risk factors for problems during labor, either method is acceptable.

Auscultation

Auscultation is done with either a special stethoscope or a device called a Doppler transducer. When the transducer is pressed against your abdomen, you can hear your fetus’s heartbeat. When auscultation is used, your ob-gyn or other health care professional will check the heart rate of the fetus at set times during labor. If you have risk factors for problems during labor or if problems appear during labor, the fetal heart rate will be checked and recorded more frequently.

Electronic fetal monitoring

Electronic fetal monitoring uses special equipment to measure the response of the fetus’s heart rate to contractions of the uterus. It provides an ongoing record that can be read. Your ob-gyn or other health care professional will review the electronic recording of the fetus’s heartbeat (called the fetal heart rate tracing) at set times. The tracing may be reviewed more frequently if problems arise.

Electronic fetal monitoring can be external, internal, or both. You may need to stay in bed during both types of electronic monitoring, but you can move around and find a comfortable position.

External fetal heart rate monitoring

External fetal heart rate monitoring method uses a device to listen to and record your baby’s heartbeat through your belly (abdomen). With this method, a pair of belts is wrapped around your abdomen. One belt uses Doppler ultrasound device to detect the fetal heart rate. The other belt measures the length of contractions and the time between them.

External fetal heart rate monitoring is often used during prenatal visits to count the baby’s heart rate. It may also be used to check the fetal heart rate during labor. The healthcare provider may also check your baby’s heart rate continuously during labor and birth. To do this, the ultrasound probe (transducer) is fastened to your belly. It sends the sounds of your baby’s heart to a computer. The rate and pattern of your baby’s heart rate are shown on a screen and printed on paper.

Internal fetal heart rate monitoring

Internal fetal heart rate monitoring method uses a thin wire (electrode) put on your baby’s scalp. The wire runs from the baby through your cervix. It is placed on the part of the fetus closest to the cervix, usually the scalp. It is connected to the monitor. This device records the heart rate. Uterine contractions also may be monitored with a special tube called an intrauterine pressure catheter that is inserted through the vagina into your uterus. Internal fetal heart rate monitoring can be used only after the membranes of the amniotic sac have ruptured (after “your water breaks” or is broken).

Internal fetal heart rate monitoring method gives better readings because things like movement don’t affect it. But it can only be done if the fluid-filled sac that surrounds the baby during pregnancy (amniotic sac) has broken and the cervix is opened. Your doctor may use internal fetal heart rate monitoring when external fetal heart rate monitoring is not giving a good reading. Or your doctor may use this method to watch your baby more closely during labor.

During labor, your healthcare provider will watch your uterine contractions and your baby’s heart rate. Your provider will note how often you are having contractions and how long each lasts. Because the fetal heart rate and contractions are recorded at the same time, these results can be looked at together and compared.

Your doctor may check the pressure inside your uterus while doing internal fetal heart monitoring. To do this, he or she will put a thin tube (catheter) through your cervix and into your uterus. The catheter will send uterine pressure readings to a monitor.

What are the risks of fetal heart monitoring?

Radiation is not used for this test. The transducer usually causes no discomfort.

You may find the elastic belts that hold the transducers in place slightly uncomfortable. These can be readjusted as needed.

You must lie still during some types of fetal heart rate monitoring. You may need to stay in bed during labor.

With internal monitoring, you may have some slight discomfort when the electrode is put in your uterus.

Risks of internal monitoring include infection and bruising of your baby’s scalp or other body part.

Note: You should not have internal fetal heart rate monitoring if you are HIV positive. This is because you may pass the infection on to your baby.

You may have other risks depending on your specific health condition. Be sure to talk with your provider about any concerns you have before the procedure.

Certain things may make the results of fetal heart rate monitoring less accurate. These include:

• Obesity of the mother
• Position of the baby or mother
• Too much amniotic fluid (polyhydramnios)
• Cervix is not dilated or the amniotic sac is not broken. Both of these need to happen to do internal monitoring

Why might I need fetal heart rate monitoring?

Fetal heart rate monitoring is especially helpful if you have a high-risk pregnancy. Your pregnancy is high risk if you have diabetes or high blood pressure. It is also high risk if your baby is not developing or growing as it should.

Fetal heart rate monitoring may be used to check how preterm labor medicines are affecting your baby. These are medicines are used to help keep labor from starting too early.

Fetal heart rate monitoring may be used in other tests, including:

• Nonstress test. This measures the fetal heart rate as your baby moves.
• Contraction stress test. This measures fetal heart rate along with uterine contractions. Contractions are started with medicine or other methods.
• A biophysical profile. This test combines a nonstress test with ultrasound.

Things that may affect the fetal heart rate during labor:

• Uterine contractions
• Pain medicines or anesthesia given to you during labor
• Tests done during labor
• Pushing during the second stage of labor

Your healthcare provider may have other reasons to use fetal heart rate monitoring.

What happens during fetal heart rate monitoring?

You may have fetal heart rate monitoring in your healthcare provider’s office or as part of a hospital stay. The way the test is done may vary depending on your condition and your healthcare provider’s practices.

Generally, fetal heart rate monitoring follows this process:

External fetal heart rate monitoring

1. Depending on the type of procedure, you may be asked to undress from the waist down. Or you may need to remove all of your clothes and wear a hospital gown.
2. You will lie on your back on an exam table.
3. Your healthcare provider will put a clear gel on your abdomen.
4. Your healthcare provider will press the transducer against your skin. The provider will move it around until he or she finds the fetal heartbeat. You will be able to hear the sound of the fetal heart rate with Doppler or an electronic monitor.
5. During labor, your healthcare provider may check the fetal heart rate at intervals or nonstop, based on your condition and the condition of your baby.
6. For continuous electronic monitoring, your healthcare provider will connect the transducer to the monitor with a cable. A wide elastic belt will be put around you to hold the transducer in place.
7. Your healthcare provider will record the fetal heart rate. With continuous monitoring, the fetal heart pattern will be displayed on a computer screen and printed on paper.
8. You may not be able to get out of bed with nonstop external fetal heart rate monitoring.
9. Once the procedure is done, the provider will wipe off the gel.

Internal fetal heart rate monitoring

1. You will be asked to remove your clothes and put on a hospital gown.
2. You will lie on a labor bed. Your feet and legs will be supported as for a pelvic exam.
3. Your healthcare provider will do a vaginal exam with a gloved hand to see how far you are dilated. This may be slightly uncomfortable.
4. If the amniotic sac is still intact, your healthcare provider may break open the membranes with a tool. You will feel warm fluid coming out of your vagina.
5. Your healthcare provider will feel the part of the baby at the cervical opening with gloved fingers. This is usually the baby’s head.
6. Your healthcare provider will put a thin tube (catheter) into your vagina. He or she will put a small wire at the end of the catheter on the baby’s scalp. He or she will gently turn it on the baby’s skin.
7. Your healthcare provider will remove the catheter and leave the wire in place on the baby’s scalp.
8. Your healthcare provider will connect the wire to a monitor cable. He or she will keep it in place with a band around your thigh.
9. You may not be able to get out of bed with nonstop internal fetal heart rate monitoring.
10. Once the baby is born, the provider will remove the wire.

What happens after fetal heart rate monitoring?

You do not need any special care after external fetal heart monitoring. You may go back to your normal diet and activity unless your healthcare provider tells you otherwise.

After internal fetal heart rate monitoring, your healthcare provider will check your baby’s scalp for infection, bruising, or a cut. The provider will clean the site with an antiseptic.

Your healthcare provider may give you other instructions, based on your situation.

What happens if the fetal heart rate pattern is abnormal?

Abnormal fetal heart rate patterns do not always mean there is a problem. Other tests may be done to get a better idea of what is going on with your fetus. If there is an abnormal fetal heart rate pattern, your ob-gyn or other health care professional will first try to find the cause. Steps can be taken to help the fetus get more oxygen, such as having you change position. If these procedures do not work, or if further test results suggest your fetus has a problem, your ob-gyn or other health care professional may decide to deliver right away. In this case, the delivery is more likely to be by cesarean birth or with forceps or vacuum-assisted delivery.

Abnormal fetal heart rate

Fetal rhythm abnormalities, which include fetal heart rates that are irregular, too fast or too slow, occur in up to 2% of pregnancies and account for 10–20% of the referrals to fetal cardiologists ³. They are usually identified by the obstetrical clinician who detects an abnormal fetal heart rate or rhythm using a Doppler “listening device” at routine assessment of the pregnant mother. While such clinical assessments begin at 12–14 weeks, most fetal arrhythmias are detected only after 20 weeks. The vast majority of affected pregnancies have isolated premature atrial contractions which may have even spontaneously resolved by fetal echocardiographic assessment. Less than 10% of referrals for fetal rhythm abnormalities have a sustained tachyarrhythmia or bradyarrhythmia considered to be of clinical significance, as they may indicate severe systemic disease or may have the potential to compromise the fetal circulation themselves. For such abnormalities prenatal diagnosis with the potential for prenatal, perinatal and neonatal intervention may be critical and may ultimately improve outcome.

Fetal heart rate abnormalities key points

• Evaluation of fetal arrhythmias is largely based on the assessment of the chronological relationship between atrial and ventricular contractions
• The majority of referrals for fetal arrhythmias represent benign atrial premature beats
• Accurate definition of the atrioventricular (AV) relationship permits delineation of the type of tachyarrhythmia and bradyarrhythmia and may assist in more appropriate management of affected pregnancies
• Most forms of fetal supraventricular tachycardia (SVT), even in the presence of hydrops, are treatable before birth through maternally administered medications
• Maternal autoantibody mediated fetal AV block and cardiomyopathy evolves as a consequence of the transplacental passage of maternal antibodies, the influence of which may be ameliorated through the use of maternally administered corticosteroids.

References

1. Pildner von Steinburg S, Boulesteix AL, Lederer C, et al. What is the “normal” fetal heart rate?. PeerJ. 2013;1:e82. Published 2013 Jun 4. doi:10.7717/peerj.82 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3678114
2. Bickhaus J, Perry E, Schust DJ. Re-examining Sonographic Cut-off Values for Diagnosing Early Pregnancy Loss. (2013) Gynecology & obstetrics (Sunnyvale, Calif.). 3 (1): 141. doi:10.4172/2161-0932.1000141
3. Hornberger LK, Sahn DJ. Rhythm abnormalities of the fetus. Heart. 2007;93(10):1294–1300. doi:10.1136/hrt.2005.069369 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2000955