Due Date (Naegele)

This formula gives an estimated due date by starting from the first day of a person’s last menstrual period and adding about 280 days, which is roughly 40 weeks. It works because pregnancy is counted from the start of the last menstrual period rather than from conception, since ovulation and fertilization are hard to date exactly in routine care. The result is only an estimate, because the true delivery date can shift by days or even weeks.

Use this when estimating a pregnancy due date in basic obstetrics, prenatal planning, or health education. It is especially useful when the last menstrual period is known but conception date is not.

• Using the date of conception instead of the first day of the last menstrual period
• Forgetting that 280 days is an estimate, not a guaranteed delivery date
• Adding 9 calendar months without checking the actual day count