Can an Early Ultrasound Be Off by About Two Weeks?

Yes, an early ultrasound can sometimes be off by about two weeks. However, the accuracy of an ultrasound as a tool for estimating gestational age depends on various factors. Ultrasounds, particularly those performed in the first trimester up to about 13 weeks of gestation, are generally considered quite accurate for dating a pregnancy. However, several factors can cause discrepancies in the results.

Factors Contributing to Ultrasound Discrepancies

Several factors can lead to inaccuracies in the estimation of gestational age through an ultrasound. These factors include irregular ovulation, the timing of the ultrasound, measurement variability, and multiple pregnancies.

Irregular Ovulation

If a woman’s menstrual cycles are irregular, the estimated due date based on the last menstrual period (LMP) may not align with the ultrasound measurements. Regular ovulation is crucial for accurate dating as it helps in determining the fertilization date.

Timing of the Ultrasound

The accuracy of dating a pregnancy through an ultrasound decreases as the pregnancy progresses. Ultrasounds performed early in the first trimester (before 9 weeks) tend to be more accurate than those performed later.

Measurement Variability

Ultrasound measurements can vary slightly due to the technician's technique or the baby's position. This can affect the accuracy of the estimated gestational age.

Multiple Pregnancies

If a woman is carrying multiples, the growth patterns may differ from a singleton pregnancy. This can also contribute to inaccuracies in the dating process.

Accuracy of Early Ultrasounds

Earlier ultrasounds are generally more accurate for estimating gestational age. New formulas have been developed to improve the accuracy of ultrasound dating, particularly at various stages of pregnancy. For example, a study published in the Obstetrics Gynecology journal in August 2017 (Volume 130, Issue 2, pages 433–441) examined the accuracy of these formulas.

Comparison of Ultrasound Formulas

The new formulas have been shown to perform better than those developed in 1984. The study found that:

One week off from 14 to 20 weeks gestation. 10 days off from 21 to 27 weeks gestation. 17 days off from 28 to 40 weeks gestation.

These accuracies are significant improvements over the older formulas, which had an estimation error of 10.4 days from 21 to 27 weeks and 17.0 days from 28 to 40 weeks. Racial and ethnic-specific formulas did not outperform the neutral formula.

Conclusion

While early ultrasounds can be off by up to two weeks, they are generally reliable for estimating gestational age, especially in the first trimester. However, healthcare providers may need to monitor the pregnancy closely if there is a significant difference between the ultrasound dating and the expected due date. Follow-up ultrasounds may be recommended to track growth and development.

Understanding the factors that can affect the accuracy of ultrasounds and the improvements in ultrasound formulas can help both healthcare providers and pregnant individuals make informed decisions about their healthcare and the pregnancy journey.