Complex sentence: Naegele's Rule is a standard way of calculating the due date for a pregnancy; simply subtract three months from the first day of the woman's last menstrual period and add seven days to that date.
Split into simpler sentences: Naegele's Rule is a standard way of calculating the due date for a pregnancy. The rule works by subtracting three months from the first day of the woman's last menstrual period and adding seven days to that date.