A pregnancy due date is the estimated delivery date — usually 280 days (40 weeks) from the first day of the last menstrual period (LMP). The calculator uses Naegele's rule and adjusts for cycle length other than 28 days.
What is Due Date?
A normal human pregnancy lasts about 40 weeks (280 days), counted from the first day of the last menstrual period — not from conception. This is the standard convention used in obstetric ultrasound, prenatal care, and birth certificates.
Naegele's rule is the formula used: add 280 days (or 9 months and 7 days) to the first day of the LMP. It assumes a regular 28-day cycle with ovulation on day 14. Women with longer or shorter cycles need an adjustment — the calculator handles this automatically when you enter your cycle length.
Only about 5% of babies are actually born exactly on the due date. The window 38-42 weeks is considered full-term. First-time mothers tend to deliver slightly later than the date predicts; subsequent children are often slightly earlier.
Naegele's rule with cycle adjustment
- LMP
- Last menstrual period—first day of the most recent period
- Cycle length
- Cycle days—average length of menstrual cycle (typically 28)
How to use this calculator
Enter the first day of your last period and your average cycle length.
Enter the LMP date
First day of the last menstrual period — when bleeding started, not when it ended.
Enter your cycle length
Most calculators default to 28 days. If yours is different (24-35 is normal), enter the actual average. Longer cycles push the due date later.
Read due date + current stage
The calculator shows the estimated due date, current pregnancy week and trimester (if you are already pregnant), and the estimated conception date.
Why know your due date
Prenatal care scheduling
Doctors schedule scans, blood tests, and check-ups at specific gestational weeks. The due date defines the timeline.
Maternity leave planning
Employers in India provide 26 weeks of paid maternity leave (Maternity Benefit Act). Plan when to start it relative to the due date.
Trimester awareness
First trimester (week 1-12), second (13-26), third (27-40). Each has different physical changes, dietary needs, and risks.
Tracking development
Pregnancy apps and books are organised by week. Knowing your week is essential for following along.
Common mistakes to avoid
Using last day of period instead of first
Use the FIRST day of bleeding. Using the last day will push due date estimates 5-7 days too late.
Ignoring an irregular cycle
If your cycles vary 25-35 days, the calculator's estimate is rough. An early ultrasound (8-12 weeks) is far more accurate than LMP-based dates.
Glossary
- LMP (Last Menstrual Period)
- First day of bleeding in the most recent menstrual cycle. The reference point for due date calculation.
- EDD (Estimated Due Date)
- The predicted date of delivery, 280 days from LMP.
- Gestational age
- How far along a pregnancy is, measured in weeks since LMP.
- Trimester
- A division of pregnancy: first (1-12 weeks), second (13-26), third (27-40).
- Term
- 37-42 weeks gestational age. Earlier = preterm; later = post-term.
- Naegele's rule
- The standard formula for calculating EDD: LMP + 280 days.
References
- WHO — Pregnancy and prenatal care— World Health Organization
- Maternity Benefit Act 1961 (amended 2017)— Ministry of Labour and Employment