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 can mean higher odds of miscarriage, but it may also be due to the pregnancy not being as far along as estimated.

Normal and Slow Fetal Heart Rates

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 beats per minute) and then gradually decrease as the fetus approaches term.

Studies show higher odds of miscarriage when the baby has a heart rate of fewer than 100 beats per minute at 6.2 weeks of pregnancy or less than 120 beats per minute at 6.3 to 7 weeks.

When Ultrasound Shows a Slow Fetal Heart Rate

If your ultrasound revealed that your baby had a slow heart rate, you are probably scared and concerned, especially if you have been searching for information on what this might mean. You may feel frustrated that you have to wait a week for a followup. But 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. There is nothing you or your physician can do to affect the ultimate outcome. When pregnancies miscarry after detecting a slow fetal heart rate, the reason is often chromosomal abnormalities that were present at conception.

Value of Ultrasound During Pregnancy

During pregnancy, ultrasound is used for many reasons including the following:

  • Dating (figuring out the age of the fetus)
  • Aneuploidy (chromosome) assessment
  • Cervical length assessment
  • Determination of fetal well-being (biophysical profile)
  • Anatomical assessment

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 following:

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

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

Unclear Heartbeat Exhibited During Ultrasound

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.

Fetal Demise During Late Pregnancy

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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