Circle of time: errors in the use of the pregnancy wheel

Despite the accuracy of pregnancy dating by ultrasound, obstetricians commonly use the pregnancy wheel for the assessment of gestational age at each visit. However, there has been limited assessment of the accuracy of the wheel, or variation among wheels. This study sought to determine the accuracy of pregnancy wheels in predicting term gestation, when entering first-trimester ultrasound-determined dating.

A selection of four pregnancy wheels of varying types were utilized to determine estimated date of delivery (EDD) based upon an ultrasound assessment of 12 0/7 weeks' gestation at random dates throughout each of the 12 months of the year. As a control, EDD was calculated as 196 days from the 12 0/7 week ultrasound assessment by a computerized program using the years 2003-04. The mean difference in days between each wheel and the computer-assessed EDD was determined. Statistical comparison was performed with Kruskal-Wallis one-way analysis of variance with post-hoc Dunnett's test.

There were marked differences between the EDD determined by wheels versus the computer. One wheel differed by an average of more than 3 days from computer-assessed EDD, with a range of + 1 to +5 days. The difference between wheels and computer-assessed EDD varied monthly.

In view of the need for accurate gestational age assessment for clinical interventions (e.g. post-dates testing) clinicians should be aware of the potential inaccuracies of pregnancy wheels.