Application of a Naegele’s Formula Adapted to the Long Count Mayan Calendar to Calculate the Expected Date of Birth During a Leap Year

Objective: To evaluate whether the Naegele formula applied to the Mayan calendar can improve the accuracy of due date prediction. Methods: A database of pregnant women who gave birth during the leap year 2016 was analyzed (post hoc analysis). The date of the last menstruation (confirmed by ultrasound) was converted to the Mayan date using an online converter. The Naegele formula was adapted to the Mayan calendar and applied to the date of the last menstruation on the Mayan calendar. The resulting Mayan calendar date was converted back to the Gregorian calendar. Discrepancies between the actual due date and that predicted by the traditional Naegele formula and by adding 280 days to the last menstruation were compared with the discrepancies obtained by applying the Mayan calendar-based formula. Results: The pregnancy duration predicted by the Mayan calendar-based formula better approximates the actual pregnancy duration than other methods in the entire database of 601 cases. In normal pregnancies, the Naegele formula deviates from the actual due date by an average of -0.982 days (±3.548), the Mayan calendar-based formula deviates from it by an average of -0.254 days (±3.481), while adding 280 days to the date of the last menstrual period deviates from it by an average of 0.247 days (±3.487) (p<0.001 for both comparisons). In further subgroup analyses, the Mayan calendar-based formula appears to be more accurate and equally precise than the other methods. Conclusions: The Naegele formula adapted to the Mayan calendar improves the estimation of the expected due date compared to the Naegele’s formula.