Your Pregnancy Matters

Understanding how 4 abnormal ultrasound findings relate to miscarriage risk

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Some abnormal ultrasound findings are associated with higher risk of pregnancy loss, but many patients go on to have successful pregnancies.

In the world of electronic medical records, patients sometimes get access to their pregnancy imaging results before they have a chance to visit with their doctor about the findings. Words such as "abnormal" or "hemorrhage" can trigger concerns about increased risks of miscarriage.

This is especially true after the first trimester ultrasound. During this exam, we measure three items that tell us whether a fetus is developing as expected:

  • Yolk sac
  • Crown-to-rump length
  • Heart rate

We also look at other features that may influence the outcome of the pregnancy, such as the presence of subchorionic hemorrhage or bleeding between the sac and the uterus.

Patients often ask, "If everything looks OK at my first-trimester ultrasound, am I out of the woods for miscarriage risk?" Unfortunately, that is never truly the case until delivery. The average pregnancy has an approximate 5% risk of miscarriage. However, if nothing unusual is detected with all four of these markers, patients likely are not at increased risk.

Not every abnormal measurement means something is wrong. For example, a smaller than expected crown-to-rump (head to bottom) number may mean your pregnancy is not as far along as you thought.

However, certain abnormal readings are concerning, especially when seen together. A 2018 study found that in pregnancies with a combination of a low fetal heart rate and a small crown-to-rump length, the risk of miscarriage increases from 5% to 21%. Still, 20% of pregnancies with both abnormal findings resulted in successful live births.

So, what do these markers mean for your pregnancy? There is no general answer – each patient should discuss her results with her doctor. In the meantime, I wanted to share an overview of these findings so you can know what to expect at your follow-up visit.

Related reading: Pregnancy without ultrasound? Pros and cons

4 abnormal ultrasound findings

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The arrows point to bleeding between the sac and the uterine wall.

1. Subchorionic hemorrhage

This condition occurs when the membranes or the sac that holds the fetus in the uterus, partially separate from the uterine wall. Blood leaks into the area between the uterine wall and the pregnancy which may be visible on ultrasound.

Subchorionic hemorrhage occurs in approximately 16% to 25% of pregnancies and is a common cause of vaginal bleeding in the first 20 weeks. Your doctor may recommend "pelvic rest" if this is seen on ultrasound and you are having any symptoms like vaginal bleeding or cramping.

This means no sex, tampon use, or douching for a while. If you are taking blood thinners for another health condition, the doctor may discuss a different medication or therapy.

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The small rings in these images are yolk sacs. On the left, the sac is normal size. On the right, it is bigger than expected.

2. Abnormal yolk sac

The yolk sac is the earliest fetal structure that forms in the gestational sac within the uterus during pregnancy.

Having a yolk sac that is too large or too small has been associated with pregnancy loss.

However, abnormal sac size occurs in approximately 17% of pregnancies. In many cases, women go on to have normal pregnancies.

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The embryo is measured between the two “+” calipers.

3. Small crown-rump length

Between six and eight weeks of gestation, the crown-rump length is the measurement of the entire length of the embryo. Smaller than expected fetuses at this gestational age may mean there is a concern for miscarriage.

However, in some cases, it means your pregnancy is just not as far along as we thought.

Small crown-rump length may be more significantly associated with increased risk of pregnancy loss in pregnancies assisted by in vitro fertilization.

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The scan shows a fetal heart rate of 82 beats per minute, which is well below normal.

4. Low fetal heart rate

In general, a low fetal heart rate is considered to be fewer than 100-120 beats per minute. This condition is also known as fetal bradycardia.

A low fetal heart rate can be an early marker of potential miscarriage

In the study, the risk of miscarriage increased from 5% to 21% if both a low fetal heart rate and small crown-rump length were present.

What happens next with abnormal findings

There is no treatment if these findings are present on your ultrasound. What’s important to remember is that not every abnormal ultrasound finding is cause for alarm.

Even in cases where a small crown-rump length and a slow heart rate were present early in pregnancy, 80% of the pregnancies developed normally.

We’ll discuss and explain the ultrasound with you and what it might mean for your pregnancy. And if you are concerned about results you receive in your EMR, don't hesitate to ask your provider questions – that's what we're here for.

Our goal with every prenatal test and exam is to help you have the best possible outcome: a happy, healthy mom and baby.

To visit with an Ob/Gyn provider, call 214-645-8300 or request an appointment online.

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