Gestational Calendars: The Wheel Deal

Obstetricians are familiar with the cardboard/paper wheels used to calculate the duration of the pregnancy and the due date. In the February issue of the American Journal of Obstetrics and Gynecology, Drs. Chambliss and Clark tested the familiar wheels against downloadable electronic apps for both 2013 and the leap year of 2012.

... The last menstrual period was set at Jan. 1, 2013, and the estimated date of confinement obtained was compared with the estimated date of confinement of Oct. 8th if the pregnancy completed 280 days. The process was performed on 20 electronic APPs downloadable to cell phones. The process was repeated for both for the leap year of 2012.

Thirty-one paper wheels from a variety of sources were collected. Ten wheels (35%) were consistent with the standard pregnancy duration of 280 days. Among the wheels surveyed, the largest discrepancy was 4 days short of 280 days. Two wheels gave an estimated date of confinement that differed from each other by 7 days. Wheels from the same source did not agree with each other. Twenty electronic gestational age calculators were examined. All 20 gave an estimated date of confinement of Oct. 8 consistent with 280 days. None of the paper gestational wheels but all of the APPs corrected for a leap year.

The authors found variability of the paper wheels compared with both electronic apps and among each other. Since many clinical decisions, eg. timing of delivery, depend on knowing the gestational age, the use of these wheels becomes problematic. Therefore, Drs. Chambliss and Clark argue for their abandonment.

There is one problem, however. The length of a term pregnancy is actually quite variable. The Early Pregnancy Study found that human gestational length varies considerably even when measured exactly (from ovulation).

These investigators offered the following:

Variability in the length of human gestation limits the ability to predict delivery dates. Nonetheless, it is common clinical practice to assign pregnant women a due date based on 280 days from the LMP. A more evidence-based approach might be to assign a range of due dates (perhaps the interquartile range, in which half of women will deliver) or to describe the due date as a median (before which half of women will deliver).

So don't throw those wheels away so fast.