Due Date Calculator
Naegele's Rule adds 280 days (40 weeks) to the first day of your last menstrual period to estimate the due date. This calculator also shows how many weeks pregnant you currently are based on days since your LMP.
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Formula
Due date = LMP + 280 days; Weeks pregnant = Days since LMP ÷ 7
Naegele's Rule dates back to the 19th century and remains the standard method for estimating gestational age. It assumes a 28-day cycle with ovulation at day 14, meaning fertilization occurs approximately 14 days after LMP. The 280-day period covers 40 weeks of gestation from LMP or approximately 38 weeks from conception. Only about 5% of babies arrive on their exact due date.
How to use the Due Date Calculator
- 1
Enter your days since last menstrual period
Value should be in days.
- 2
Read your results instantly
Results update in real time as you type.
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Understanding your due date
The due date calculated by Naegele's Rule is an estimate, not a deadline. Full-term pregnancy ranges from 37 to 42 weeks — any birth within this window is considered term. Approximately 80% of babies are born within two weeks of the estimated due date. First-time mothers statistically deliver slightly later than the due date on average. Your healthcare provider may adjust the due date based on first-trimester ultrasound measurements, which are more accurate than LMP dating when cycles are irregular.
Gestational age versus fetal age
Gestational age is measured from the LMP, not from conception. Because conception typically occurs 14 days after LMP, gestational age is always about 2 weeks more than fetal age (time since conception). When your provider says you are 8 weeks pregnant, the embryo/fetus is actually about 6 weeks old. This convention is universal in obstetrics and is the basis for all standard prenatal milestones and trimester divisions.
Tips & Insights
Record the exact LMP date
The accuracy of your estimated due date depends entirely on knowing the precise first day of your last menstrual period. Note it immediately if you are trying to conceive.
First-trimester ultrasound is the most accurate dating method
Crown-rump length measured by ultrasound at 6–10 weeks is more accurate than LMP dating, particularly for women with irregular cycles. If the ultrasound date differs from LMP dating by more than 7 days, providers typically adjust to the ultrasound date.
Discuss the 'due window' with your provider
Rather than fixating on a single due date, discuss the 38–42 week window. Post-dates monitoring typically begins at 41 weeks in most guidelines.
Worked Examples
LMP was 7 days ago
Due in 273 days (≈ 1 week pregnant)
LMP was 70 days ago
Due in 210 days (10 weeks pregnant)
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Frequently Asked Questions
How accurate is Naegele's Rule?
It is accurate on average but assumes a regular 28-day cycle. Women with longer or shorter cycles may have actual ovulation dates that shift the true due date by days to over a week.
Can I calculate my due date from conception date?
Add 266 days (38 weeks) to your conception date. The conception-date calculator on this site provides this calculation directly.
What is the difference between trimesters?
First trimester: weeks 1–12. Second trimester: weeks 13–26. Third trimester: weeks 27–40. These boundaries are conventionally based on gestational age from LMP.
Does a C-section change the due date?
The estimated due date remains the same. Planned (elective) C-sections are typically scheduled for 39 weeks; the due date is still calculated from LMP.
Can IVF pregnancies use this calculator?
IVF pregnancies use a different dating convention: gestational age is calculated from the egg retrieval or transfer date using fixed offsets. This LMP-based calculator is not appropriate for IVF dating.
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