Risk of Miscarriage With Slow Fetal Heartbeat

Causes and Consequences of Fetal Bradycardia

When the fetal heart rate is slower than expected during the first trimester, your physician may note that there's some cause for concern and recommend a follow-up ultrasound to check the baby's development. A slower than expected fetal heartbeat, referred to as fetal bradycardia, can mean higher odds of miscarriage. But it also may also be due to the pregnancy not being as far along as estimated.

Miscarriage Risk

The baby's heart rate should start somewhere around 100 beats per minute (bpm) around 6 weeks gestation (the time of the first detection), peak at 9 weeks (sometimes even reaching levels as high as 180 bpm) and then gradually decrease as the fetus approaches term.

The average fetal heart rate changes during pregnancy. Some doctors consider the lower limit of normal to be:

  • 100 bpm up to 6.2 weeks of gestation
  • 120 bpm at 6.3 to 7 weeks of gestation

If the fetal heart rate drops below this during the first seven weeks, the risk of miscarriage is seen to increase, with slower rates corresponding to poorer survival.

A study published in the Journal of Ultrasound in Medicine suggests a 64 percent risk of miscarriage when the fetal heart rate is less than 90 beats per minute (bpm) on or before 6.2 weeks. If the heart rate is below 100 bpm between 6.3 and 7 weeks, there is a 0 percent chance of survival.

If your ultrasound revealed that your baby had a slow heart rate, you will be probably scared, especially if you have to wait a week for a followup. Unfortunately, there is no way to tell what is happening without that wait. Sometimes the baby's heart rate will normalize, and then the pregnancy will continue without further complications.

But sadly sometimes the outcome goes the other way. If fetal bradycardia is diagnosed, there is little you or your doctor can do to affect the outcome. Persistent fetal bradycardia is often the result of a chromosomal abnormality that makes it difficult, if not impossible, to bring the pregnancy to term.

Ultrasound Uses

During pregnancy, an ultrasound (which produces internal images using sound waves) may be used for a variety of different reasons:

  • Dating (figuring out the age of the fetus)
  • Aneuploidy (chromosomal) assessment
  • Cervical length assessment
  • Evaluation of fetal well-being
  • Assessing anatomical development

During 13 to 18 weeks of gestation, the age of the fetus can be determined using ultrasound. Different measurements are used to estimate the baby's age including the head circumference, abdominal circumference, length of the femur (thigh bone), and biparietal diameter (diameter of the skull).

The accuracy of dating is plus or minus 7 days. In other words, the baby's age is estimated within about a week. At 24 weeks, this accuracy decreases and ultrasound is best used to assess fetal weight and growth.

Using an ultrasound, fetal heart tones can be heard at around 10 weeks of gestation. It takes between 18 and 20 weeks to hear the baby's heartbeat using a fetoscope.

Abnormal Ultrasound Findings

Ultrasound is the gold standard to determine whether the fetus is alive. Sadly, if a fetus is present but no heartbeat is detected, then the fetus has died.

Sometimes during the first trimester of pregnancy, it's unclear whether there's a heartbeat. In these cases, further tests need to be performed to figure out whether the baby is alive. These tests include serial beta hCG levels, a type of pregnancy hormone.

It is possible that a slow fetal heart rate is only temporary. Known as transient bradycardia, it can occur when an intrauterine ultrasound creates excessive pressure in the uterus and temporarily slows the heart rate.

During late pregnancy, the first sign of fetal demise is usually lack of movement. When the baby isn't moving, ultrasound can be used to detect fetal heart tones and uncover a reason for the lack of fetal movement.

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