Risk of Miscarriage With Slow Fetal Heartbeat

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 be due to the pregnancy not being as far along as estimated.

Miscarriage Risk

The average baby's heart rate is measured around 110 beats per minute (bpm) around 6 weeks gestation (the time of the first detection may be as early as 5 weeks gestation), peaks at 9 weeks (sometimes reaching levels near 180 bpm), and then gradually decreases as the fetus approaches term.

It is completely normal for the fetal heart rate to vary throughout pregnancy. The accepted lowest beats per minute for a normal heart rate are:

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

If the fetal heart rate is lower than these limits during the first 7 weeks, the risk of miscarriage is seen to increase. It is possible that a slow fetal heart rate in the first trimester is only temporary. Known as transient bradycardia, it can occur when a transvaginal ultrasound creates excessive pressure in the uterus and temporarily slows the heart rate.

A study published in the American Journal of Roentgenology examined if clinicians could determine which embryos with low heart rate at 6 weeks gestational age were at-risk and which ones were not.

Though further research is needed, embryos with below normal heart rates who were also small for gestational age had the greatest burden of risk. Fourteen of 18 pregnancies ended in miscarriage. While 16 of 18 pregnancies with normally developing embryos and a developing heart rate below the accepted normal range concluded in normal births.

Small for gestational age measurements in the first trimester can be a sign of poor development, which may indicate genetic abnormalities, which is the primary cause of miscarriage in the first trimester.

If your ultrasound revealed that your baby had a slow heart rate, you will probably be scared, especially if you have to wait a week for a follow-up. 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:

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

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.

Abnormal Ultrasound Findings

Ultrasound is the standard procedure to determine the status of the fetus.

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 determine the status of the fetus. These tests include serial beta hCG levels, a type of pregnancy hormone.

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.