3 Reasons Why a Fetal Ultrasound May Be Wrong

Why a vital diagnostic tool can sometimes be inaccurate

Pregnant Woman Getting Ultrasound
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The use of ultrasound in pregnancy is fairly commonplace. It was originally designed to help monitor the status of complicated or high-risk pregnancies. Today, ultrasounds are considered a standard facet of prenatal care.

While ultrasounds can provide doctors and midwives is valuable insights into how a pregnancy is progressing, there are times when the results may either be misleading or inaccurate.

Among some of the more common reasons for this:

An Incorrect Pregnancy Date

An ultrasound technician, known as a sonographer, will look for certain features during the different stages of a pregnancy to determine if things are proceeding as they should. If the technician is unable to find that feature, it may be the sign of a problem. Or not.

For example, if a woman is seven 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: the dating of the pregnancy is off, and you're not nearly as far along as you thought.

In such case, the doctor or midwife may order another ultrasound in a week. In the end, the pregnancy could be just fine, and all that is really needed a simple recalibration of the date.

Technician Error

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 actual research into this effect in obstetrics, a study into the use of ultrasound in an emergency setting showed that errors or missed diagnoses occurred in anywhere from eight to 10 percent of cases.

Similar results were seen other techniques such as chest X-rays (where the "miss rate" was over 20 percent) and mammography (where the "miss rate" was as high as 75 percent).

If there is ever any uncertainty about the competency of a sonographer, you should ask for the attending physician to be present during the exam.

Obesity

Being overweight can make it difficult—and, in some cases, impossible—for a technician to get a clear ultrasound image. This is especially concerning since obesity is associated with an increased risk of fetal birth defect (including heart and gastrointestinal abnormalities) and such pregnancy complications as pre-eclampsia and postpartum hemorrhage.

Studies have shown that obesity (defined as a body mass index of over 30kg/m2) reduces the likelihood of an accurate reading by nearly 50 percent (37 percent versus 19 percent) compared to women of normal weight.

To overcome this, sonographers will often perform a transvaginal ultrasound (a device inserted into the vagina) at 12 to 15 weeks of gestation. This is the period during which defects can more often be spotted.

In all other cases, it is important that the technician be experienced in knowing how to "work around" areas of excess fat when performing an external, abdominal ultrasound.

Sources:

Paladini, D. "Sonography in obese and overweight pregnant women: clinical, medicolegal and technical issues." Ultra Obstet Gyne. 2009; 33(6):720-729.

Pinto, A.; Pinto, F.; Faggian, A. et al. "Sources of error in emergency ultrasonography." Crit Ultrasound J. 2013; 5(Suppl 1): S1.

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