Pregnancy Due Date Calculator
The Pregnancy Due Date Calculator estimates your baby's due date based on your last menstrual period (LMP) or conception date. The standard method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last period. The due date is an estimate — only about 5% of babies are born on their exact due date. Most births occur within two weeks before or after the due date.
Formula
Naegele's Rule:
Due date = LMP + 280 days
= LMP + 9 months + 7 days
From conception date:
Due date = conception date + 266 days
Trimesters:
- First: weeks 1-13
- Second: weeks 14-27
- Third: weeks 28-40 Example
LMP: 2026-01-01 Due date: 2026-01-01 + 280 days Due date: 2026-10-08 Current week (as of 2026-09-09): ~36 weeks Trimester: Third Days remaining: ~29
How to Use
- Enter the first day of your last menstrual period
- Or enter the conception date if known
- The calculator estimates your due date
- Review your current week and trimester
- Consult your healthcare provider for prenatal care
Frequently Asked Questions
How accurate is the due date?
The due date is an estimate. Only 5% of babies are born on their exact due date. Most births occur between 38 and 42 weeks. First babies often arrive a few days late. Ultrasound measurements in the first trimester provide the most accurate due date.
What is Naegele's rule?
Naegele's rule, developed in the 1800s, calculates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14. It is less accurate for longer or shorter cycles.
Can I calculate the due date from conception?
Yes. If you know the exact conception date (e.g., from IVF or ovulation tracking), add 266 days. This is more accurate than LMP-based calculation because it does not depend on cycle length assumptions.
What are the three trimesters?
First trimester: weeks 1-13 (organ development). Second trimester: weeks 14-27 (growth and movement). Third trimester: weeks 28-40 (weight gain and maturation). Full term is 39-40 weeks.
What if I do not know my last period date?
An ultrasound in the first trimester (8-12 weeks) can estimate the due date by measuring the baby's size. This is the most accurate method when LMP is unknown. Later ultrasounds are less accurate for dating.