How to Use This Calculator
- Select your LMP date: Choose the calendar day bleeding began during your most recent normal period.
- Adjust your typical cycle duration: Standard cycles average 28 days; adjust if yours is typically shorter or longer.
- Track gestational progress: See your current developmental trimester and projected delivery date.
Formula and Calculation Steps
Naegele's Rule:
Estimated Due Date = LMP + 280 Days + (Cycle Length − 28 Days)
Gestational Age = Elapsed Days from LMP ÷ 7 (Weeks + Days)Worked Examples and Reference Table
| Trimester | Gestational Weeks | Key Development Milestones |
|---|---|---|
| First Trimester | Weeks 1 – 13 | Organogenesis, heart tube beats, limb buds form |
| Second Trimester | Weeks 14 – 27 | Anatomy scan, rapid skeletal growth, fetal movement felt |
| Third Trimester | Weeks 28 – 40+ | Lung maturation, rapid weight gain, head engagement |
Frequently Asked Questions
How is an estimated due date (EDD) calculated?
Standard clinical dating uses Naegele’s rule: 280 days (40 weeks) are added to the first day of your last menstrual period (LMP), adjusted for cycle lengths different from 28 days.
How many babies are born on their exact due date?
Only approximately 4% to 5% of babies are born on their exact estimated due date. Most full-term births occur within a two-week window before or after (37 to 42 weeks).
What are the three trimesters of pregnancy?
The first trimester spans weeks 1 through 13. The second trimester spans weeks 14 through 27. The third trimester begins at week 28 and continues through delivery.
What if my menstrual cycles are irregular?
If your cycles are irregular or longer/shorter than 28 days, an early first-trimester dating ultrasound (crown-rump length measurement) provides the most precise due date estimation.
Can the due date change during pregnancy?
Healthcare providers may update an initial LMP due date if an early 8-to-12-week ultrasound differs by more than 5 to 7 days from the calendar calculation.