A normal heart rate for your fetus is an essential indicator of the well being of the fetus during pregnancy, labor, and delivery. An abnormal pattern, such as a high fetal heart rate or low fetal heart rate, can warn doctors and midwives a baby is under medical stress or at risk of dying.
Normal Rate and Pattern
The heart rate of the fetus has the following characteristics:
- The baseline heart rate ranges between 110 to 160 beats per minute (BPM), according to Johns Hopkins Medicine.
- From one beat to the next, the rate can vary by 5 to 25 BPM - referred to as the normal "beat-to-beat variability."
- The baseline rate responds to changes in the mother, the uterus, the placenta, umbilical cord, or the fetus:
- If the fetus is healthy, his heart rate briefly increases when he moves or when he is stimulated, for example by uterine contractions or external sound.
- The heart rate pattern changes when the fetus is unhealthy or if he is in distress, for example, from insufficient oxygen supply (hypoxia).
- The baby's heart rate can be easily distinguished from the mother's because it is faster than hers - normal adult heart rate ranges from 60 to 100 BPM.
Monitoring Heart Rate
There are several methods doctors use to detect the fetal heartbeat and rate for routine and high-risk pregnancies.
The fetal heartbeat is first detectable and measurable on ultrasound at about five to six weeks from the last menstrual period. When there are complications during the first weeks of pregnancy, such as vaginal bleeding, a visible heartbeat on prenatal ultrasound at five to six weeks is the first confirmation the fetus is viable.
Hand-Held Fetal Doppler
A hand-held fetal Doppler is a battery-operated Doppler device that uses ultrasound waves to listen to the fetal heartbeat. It can measure the heart rate through the mother's abdomen from about 10 to 12 weeks.
The fetal Doppler is the method most often used during prenatal visits from the first trimester on. The fetal Doppler can also be used for intermittent monitoring during labor.
The external monitor is also a Doppler ultrasound device. The electronic fetal monitoring equipment is used to measure and track the heart rate during labor or during prenatal fetal surveillance testing.
An ultrasound probe (transducer) is strapped to the mother's belly and transmits the fetal heart sounds to a computer, which can provide a continuous printed tracing of the results. A separate device, also strapped to the belly, simultaneously measures uterine contractions.
Internal Fetal Scalp Electrode
An internal fetal scalp electrode is used during labor if improved or closer monitoring is needed. A thin monitoring wire (electrode) is attached to the skin of the fetal scalp by passing it through the cervix after breaking the bag of waters if it is not already broken. As with the external device, the electrode is attached to a computer that monitors and displays heart rate.
Other listening devices include the fetal stethoscope, or fetoscope. It is usually used during prenatal visits if a hand-held fetal Doppler is not available. An ordinary doctor's stethoscope might suffice to listen to the fetal heart when no other device is available.
Fetal Heart Rate by Trimester
As noted previously, a normal fetal heart rate ranges between 110 to 160 BPM, however, this range can vary depending on the trimester.
The fetal heart beat can be first visualized with ultrasound on an embryo that is about 5 to 6 weeks gestational age. The fetal heart rate at this stage is typically 100 to 120 BPM. Keep in mind that the embryo is only 3 to 7 mm in size at this time, since the embryo is so small it is normal for the heart rate to be at the lower end of the range.
As the baby grows over the next few weeks, the heart rate will gradually increase. By 10 weeks gestational age, the fetal heart rate will range between 120 to 170 BPM.
In the second trimester, the baby's heart rate tends to be around 140 to 150 BPM. A study demonstrated that the mean fetal heart rate of babies that were 20 to 24 weeks gestational age was 148 BPM.
However, it is normal for the heart rate to fluctuate in the second trimester. This will correlate with how active or sleepy the baby is at the time the heart rate is taken. The typical range is 120 to 160 BPM.
The range for the fetal heart rate in the third trimester is similar to the second trimester at 120 to 160 BPM. However, later in the third trimester it is normal for the baby's heart rate to slow down to around 130 bpm.
It is not unusual for a baby to have a low fetal heart rate in the third trimester compared to the first and second trimesters. This is due to baby's rapid growth in the third trimester. While the baby should still be active, he or she will be more crowded and restricted causing a change in baby's movements. Big rolls, punches and kicks are likely to turn into stretches and body shifts. When this happens, it is normal for the heart rate to be affected as well. However, if the baby's heart rate is consistently out of the normal range (too high or too low), this may be a sign something is wrong such as fetal distress or a heart condition.
Abnormal Rate and Patterns
During pregnancy, labor, and delivery, an abnormal heart rate that is too low, too high, or does not vary from beat to beat might indicate a fetus in trouble, or there may be a more benign cause. An American Family Physician article reviews the abnormalities doctors take into account during pregnancy and labor to decide on what to do next for mother and fetus.
Each trimester has its own specific concerns when it comes to abnormal heart rates and patterns (arrhythmia).
- In the first trimester a low fetal heart rate (bradycardia) may indicate a possible impending miscarriage due to fetal demise.
- In the second trimester, depending on the consistency and extent of the arrhythmia, there may be a concern of a possible structural issue with the heart. However, some arrhythmias in the second trimester can be normal since the electrical pathways of the heart are developing at this time. This is typically temporary and will resolve on its own.
- In the third trimester, there is a chance that an arrhythmia may indicate a problem such as fetal distress or a possible heart condition.
Bradycardia means a persistent slowing of the mean baseline fetal heart rate to less than 110 to 120 BPM. The bradycardia is more often a sign of fetal distress if it is below 100 BPM, especially if there are other abnormal heart rate patterns, such as poor beat-to-beat variability. Reasons for low fetal heart rate can vary. Causes of fetal bradycardia include:
- The position of the fetus
- Fetal congenital heart and other abnormalities
- Cord compression or cord prolapse that decrease oxygen supply to the fetus
- Other fetal compromises that cause severely low oxygen - a high risk or sudden fetal death
This is a persistent heart rate faster than 160 BPM. High fetal heart rates are not always a sign of fetal distress if there is good beat to beat variability. A rate above 180 BPM is considered a sign of a fetus in distress. Causes of fetal tachycardia include:
- Maternal anxiety and increased physical activity
- Drugs given to the mother, such as terbutaline, used to stop preterm labor
- Maternal illness including anemia, infection, and fever
- Intrauterine infection affecting the fetal membranes (chorioamnionitis) and the fetus
- Fetal anemia or abnormal fetal heart rhythms and insufficient oxygen
Poor Beat-to-Beat Variability
A flattening of the normal beat-to-beat variation of the heart rate to less than five BPM can indicate fetal distress. The causes can include:
- A preterm fetus less than 28 weeks gestation
- A fetus in distress from various causes including low oxygen, congenital fetal anomalies, or preterm labor
- Steroids used to mature the fetal lung if there is preterm labor or preterm rupture of membranes
- Magnesium sulfate used to treat preeclampsia or stop labor
- Narcotics for pain during labor
- Normal fetal sleep cycles
No Response to Fetal Stimulation
The heart rate responds to stimulation, such as uterine contractions or fetal movement, with an abrupt rise of 15 BPM or more lasting less than two minutes. A lack of response to such stimulation can indicate a sleeping or compromised fetus.
No accelerations for more than 80 minutes is a sign of a severely affected fetus at high risk for dying. Note that a premature fetus of less than 28 weeks gestation might not show a normal heart rate response to stimulation.
Heart Rate Decelerations
A deceleration is a decrease in the heart rate of 15 BPM or more, associated with a uterine contraction. The timing of the deceleration in relation to the onset and end of the contraction (early, late, variable, or prolonged) can indicate whether the fetus may or may not be at high risk for dying. The causes of concerning late, variable, or prolonged decelerations include:
- Compression of the umbilical cord, including a cord wrapped around the neck of the fetus or a knot in the cord
- Prolapse of the cord through the cervix
- A fetus with intrauterine growth restriction (IUGR)
- Compression of the fetal head
- Placental abruption - a separation of the placenta from the uterine wall
- Placental insufficiency - poor function of the placenta
- Severe or frequent contractions of the uterus
- Maternal factors, such as low blood pressure, preeclampsia, gestational diabetes
Fetal Heart Rate and Gender Prediction
The ability to predict a baby's gender by its heart rate during pregnancy is one of the many old wives tales that's been around for ages. Studies have shown that there is no evidence that this popular pregnancy folklore is true.
Ask Your OB Providers For Details
Changes in the normal heart rate and pattern help obstetrics healthcare providers assess if a fetus is at risk for a poor outcome during pregnancy and labor. Ask your doctor or midwife to explain any concerning changes in your baby's heart rate during your prenatal visits and labor.