Can an Ultrasound Scan Lie About the Weeks of a Pregnancy?

When a pregnant woman has an ultrasound, she understandably assumes that the results will be reliable. Nevertheless, there have been ongoing doubts and uncertainties: Could an ultrasound be inaccurate when determining the weeks of a pregnancy? 

Although it is not possible for an ultrasound scan to deliberately provide false information, it should be acknowledged that these scans can occasionally be inaccurate. 

Ultrasound for pregnancy

Estimating Gestational Age

To begin with, let’s acquire knowledge about the process of an ultrasound scan. This method of medical imaging is used to measure the fetus’s dimensions in order to determine the stage of pregnancy. Nonetheless, it is important to note that each baby grows at a unique pace, particularly in the initial three months of pregnancy. 

Consequently, the ultrasound’s estimation of gestational age may have a margin of error of one to two weeks. In other words, if the ultrasound indicates a ten-week pregnancy, the actual gestational age could range from eight to twelve weeks. Therefore, it is not the fault of the ultrasound itself, but rather a matter of how accurately or precisely the gestational age is estimated based on an average rate of growth. 

Ultrasound ScanPossible Variation of Gestational Age
10 Weeks8-12 Weeks
20 Weeks19-21 Weeks
30 Weeks29-31 Weeks

Factors Influencing Ultrasound Accuracy

There are various factors that can impact the precision of an ultrasound scan in determining the duration of pregnancy:

  1. Timing of the Scan: Ultrasound scans conducted in the first trimester are generally more accurate than those done in the second or third trimester. As pregnancy progresses, individual variations in growth become more pronounced.
  2. Technician Skill and Experience: The skill and experience of the ultrasound technician can greatly affect accuracy. Expertise is required to correctly position and calibrate the equipment for precise measurements.
  3. Quality of Equipment: High-tech, advanced ultrasound equipment will be more accurate than outdated or poorly maintained machines.


Although an ultrasound scan may not give an exact determination of the number of weeks into your pregnancy, it is not intentionally providing false information. Instead, any potential inaccuracies can be attributed to several factors such as different growth rates among fetuses, the timing of the ultrasound, and the technology used. It is highly recommended for expectant mothers to openly discuss their concerns with healthcare providers and acknowledge that an estimated due date is just an approximation. 

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