Enter the first day of the last menstrual period into the calculator to determine the expected date of delivery.
Medical note: This calculator provides an estimate and is not medical advice. Confirm pregnancy dating and due date with your obstetric provider; early ultrasound dating may be more accurate than last-menstrual-period dating.
- Estimates may be less accurate with irregular/long/short cycles, uncertain LMP, recent pregnancy (including miscarriage) or postpartum/breastfeeding, hormonal contraception, PCOS, or fertility treatment (e.g., IVF/embryo transfer).
- Conception/ovulation dates are often uncertain unless you have ovulation tracking or fertility treatment dates.
- Most births do not occur on the exact due date; delivery commonly occurs within a range around the estimated date.
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Naegele's Rule Formula
The following formula is used to calculate the expected date of delivery using Naegele's Rule.
EDD = LMP + 1 year - 3 months + 7 days
Variables:
- EDD is the expected date of delivery
- LMP is the first day of the last menstrual period
To calculate the expected date of delivery, add one year to the first day of the last menstrual period, then subtract three months. Finally, add seven days to the result. This method estimates due date by assuming a 28-day menstrual cycle with ovulation around day 14; if your cycles are longer/shorter or ovulation timing differs, the estimate may be off. Pregnancy length varies, so the due date is an estimate rather than an exact date.
What is a Naegele's Rule?
Naegele's Rule is a standard method used by obstetricians to estimate a pregnant woman's due date. The rule calculates the expected date of delivery by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period. It is based on an average 28-day cycle with ovulation around day 14; if cycle length or ovulation timing differs, results can vary.
How to Calculate Naegele's Rule?
The following steps outline how to use Naegele's Rule to calculate the expected date of delivery (EDD).
- First, determine the first day of the last menstrual period (LMP).
- Next, add 1 year to the LMP.
- Then, subtract 3 months from the previous step.
- Finally, add 7 days to the result to calculate the EDD.
Example Problem:
Use the following variables as an example problem to test your knowledge.
LMP: January 15, 2022
EDD = LMP + 1 year - 3 months + 7 days
