Pregnancy Due Date Calculator (LMP Method)
Use your first day of last menstrual period (LMP) to estimate your due date. You can also adjust for cycle length if your cycle is shorter or longer than 28 days.
How a due date calculator based on LMP works
A due date calculator based on LMP uses the first day of your last menstrual period to estimate your expected delivery date. This is the same starting point used in many clinics because it is usually easier to identify than the exact date of conception.
The standard approach assumes a 28-day menstrual cycle, with ovulation around day 14. From that model, pregnancy length is estimated at about 280 days (40 weeks) from LMP.
The formula (Naegele's Rule)
The classic method is often called Naegele's Rule:
- Take the first day of your LMP
- Add 1 year
- Subtract 3 months
- Add 7 days
This calculator uses the equivalent day-based formula so it can also adjust for different cycle lengths.
Why cycle length matters
Not everyone has a 28-day cycle. If your cycle is longer, ovulation usually happens later, and your estimated due date may shift later. If your cycle is shorter, your estimated due date may shift earlier.
- Longer cycle than 28 days: due date is adjusted later.
- Shorter cycle than 28 days: due date is adjusted earlier.
What your results include
After calculation, you will see:
- Estimated due date (EDD)
- Estimated conception date
- Current gestational age in weeks and days
- Days remaining until due date (or days past due date)
- Approximate trimester transition dates
How accurate is an LMP due date?
It is an estimate, not an exact prediction. Many healthy pregnancies deliver between 37 and 42 weeks, and only a small percentage of babies are born on the exact due date.
Accuracy can be affected by irregular periods, uncertain LMP date, recent hormonal contraception, breastfeeding, and conditions that affect ovulation timing. In early pregnancy, an ultrasound can provide a more precise gestational dating reference when needed.
When to contact your healthcare provider
- If your cycles are very irregular
- If you are unsure about your LMP date
- If your ultrasound date differs significantly from LMP dating
- If you have bleeding, severe pain, or other concerning symptoms
Frequently asked questions
Is this calculator for IVF pregnancies?
IVF pregnancies are usually dated from embryo transfer date and embryo age rather than LMP. Ask your fertility clinic for the most accurate expected due date.
Can I use this with irregular periods?
You can, but the result may be less accurate. If your cycles vary a lot, early ultrasound dating is especially important.
Does a due date mean I will deliver that day?
Not necessarily. The due date is a clinical target used for monitoring. Labor can begin naturally before or after that date.