In order to determine a pregnant client's estimated date of delivery using Nagele's rule, what is the first step?

The first step in using Nagele's rule to estimate a pregnant client's due date is to determine the first day of their last menstrual period (LMP). This date is crucial because Nagele's rule calculates the estimated date of delivery based on the average length of a pregnancy from the last known menstrual cycle.

Once the first day of the LMP is established, other calculations can follow, such as subtracting three months and adding a year (if necessary), and then adding seven days to arrive at the estimated date of delivery. The accuracy of the due date is highly dependent on knowing the precise LMP as it serves as the starting point for all subsequent calculations.