Reasons Why Prenatal Ultrasound Results Can Be Wrong

The use of ultrasound during pregnancy is fairly commonplace in the US. The practice was originally designed to help monitor the status of complicated or high-risk pregnancies. Today, ultrasounds are considered a standard facet of routine prenatal care.

While ultrasounds can provide doctors and midwives with valuable insights into how a pregnancy is progressing, there are times when the results can be either misleading or inaccurate. For instance, some parents might not actually get an accurate reading of their baby's size from an ultrasound alone. Here are three reasons why an ultrasound may be wrong.

Incorrect Estimated Due Date

An ultrasound technician, known as a sonographer, will look for certain features during the different stages of a pregnancy to determine if the pregnancy is proceeding as it should, which includes looking at whether or not the fetus is developing normal for its gestational age. If the technician is unable to find that feature, it may be a sign of a problem—or not.

For example, if according to their estimated due date, a person is eight weeks pregnant and the ultrasound does not reveal a fetal heartbeat, there may be a moment of panic but the explanation could actually be quite simple. It's possible that the dating of the pregnancy is off and the pregnancy is not as far along as originally thought.

In these cases, the prenatal health care provider may order another ultrasound in about a week. In the end, the pregnancy could be just fine and all that was really needed was a simple recalibration of the due date.

Technician Skill

Ultrasound technology has been greatly simplified in the past decade but still requires skill to obtain an accurate result. While most technicians have the necessary training to perform an exam, some, quite simply, are better or more experienced than others.

While there has been no dedicated research into the effect of ultrasound technician skill in obstetrics, an older study into the use of ultrasound in an emergency setting showed that errors or missed diagnoses occurred in anywhere from 8% to 10% of cases.

If you are ever feeling doubtful during your ultrasound or ultrasound technician, you can always address any concerns you have with your OB/GYN or your midwife in case a followup scan needs to be conducted.

The Pregnant Person's BMI

Because adipose tissue (aka body fat) impacts how sound travels, excess body fat can affect an ultrasound's ability to pick up sound waves and create a clear image. As a result, people with obesity may be at higher risk of inaccurate ultrasound results. Studies have shown that obesity (defined as a body mass index of over 30) reduces the likelihood of an accurate reading compared to people with a lower amount of body fat.

To overcome this concern, sonographers may perform a transvaginal ultrasound (during which the ultrasound wand is inserted into the vagina) as opposed to an abdominal ultrasound at around 12 to 15 weeks of gestation on pregnant people with obesity. This is the period during which certain congenital disorders can more often be spotted and the transvaginal transducer allows for a clearer picture as it gets closer to the uterus and developing fetus.