HEALTH · DUE DATE CALCULATOR (LMP METHOD)
Due Date Calculator (LMP Method)
Estimate your pregnancy due date from your last menstrual period (LMP) using Naegele's Rule. Includes gestational age in weeks and days, trimester boundaries, and cycle-length adjustment.
Based on Naegele's Rule (EDD = LMP + 280 days, adjusted for cycle length). This is an estimate — only your obstetric provider can confirm your due date with ultrasound dating.
About This Calculator
The LMP method is the primary clinical approach to estimating a pregnancy due date. Enter the first day of your last menstrual period and your average cycle length. The calculator applies Naegele's Rule with a cycle-length adjustment for cycles shorter or longer than 28 days, producing your estimated due date along with current gestational age (weeks and days) and first and second trimester milestones.
How It Works
Enter the first day of your last menstrual period and your average cycle length (default 28 days, valid range 20–45 days). The calculator adds 280 days to your LMP, then adjusts by (cycle length − 28) days for non-standard cycles. The result is your Estimated Due Date (EDD). Gestational age is calculated as days elapsed since LMP divided by 7, expressed as weeks and days.
The Formula
EDD = LMP + (280 + (cycleLength − 28)) days
- LMP
- First day of last menstrual period
- cycleLength
- Average cycle length in days (default 28)
Frequently Asked Questions
- How accurate is the LMP method?
- The LMP method provides an estimate, not an exact date. Gestational age is most accurately confirmed by first-trimester ultrasound (crown-rump length between 8 and 14 weeks). Only about 4–5% of babies are born on the estimated due date; most births occur within two weeks of it. Your obstetric provider may adjust the EDD based on ultrasound findings.
- What does gestational age mean?
- Gestational age is measured from the first day of your last menstrual period, not from conception. Conception typically occurs around day 14 of a 28-day cycle, so a newly fertilised egg is technically "2 weeks" gestational age. This convention is universal in obstetric practice.
- Why does cycle length affect the due date?
- Naegele's Rule assumes a 28-day cycle with ovulation on day 14. For a shorter cycle (e.g. 25 days), ovulation occurs earlier and the due date is adjusted back by 3 days. For a longer cycle (e.g. 35 days), ovulation occurs later and the due date moves forward by 7 days. The adjustment is (your cycle length − 28) days.
- What are the trimester boundaries?
- The first trimester runs from conception through the end of week 13 (LMP + 91 days). The second trimester covers weeks 14–26 (LMP + 92–182 days). The third trimester spans weeks 27–40 (LMP + 183–280 days). These are approximate boundaries used in clinical planning.