Pregnancy Due Date Calculator
Estimate your due date based on your last menstrual period
What Is the Pregnancy Due Date Calculator?
This calculator estimates your due date using Naegele's Rule, a standard obstetric formula that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP).
Naegele's Rule has been the standard bedside method for estimating a due date for well over a century, valued for how little information it needs — just one date. It works by counting pregnancy in weeks from the LMP rather than from the actual date of conception, since most people know their period start date with far more confidence than their exact ovulation or conception date. Doctors typically treat this as a starting estimate, later refined or confirmed with an ultrasound dating scan at the first prenatal visit.
If you're not yet pregnant but tracking your cycle, the Ovulation Calculator uses the same LMP-based approach to estimate your fertile window. And since fluid needs typically rise during pregnancy, the Water Intake Calculator can help you think through a baseline daily hydration target.
Pregnancy Due Date Calculator Formula
Estimated Due Date = LMP + 280 days
How Is the Pregnancy Due Date Calculator Calculated?
Naegele's Rule assumes a typical 28-day menstrual cycle with ovulation around day 14, and pregnancy lasting about 280 days (40 weeks) from the first day of the last period — not from conception. It's a widely used clinical approximation rather than a precise prediction.
Because the rule assumes a 28-day cycle, it's most accurate for people whose cycles are close to that length. Shorter or longer natural cycles shift the actual date of ovulation earlier or later than day 14, which means the true conception date — and therefore the truly "due" date — can differ from this LMP-based estimate. That's part of why an early ultrasound, which measures the embryo or fetus directly, is often used to confirm or adjust the date calculated from LMP alone.
Pregnancy Due Date Calculator Example
If your last menstrual period started on January 1, adding 280 days gives an estimated due date of roughly October 8 of the same year.
If your LMP started on April 15, adding 280 days gives an estimated due date around January 20 of the following year — a useful reminder that due dates often land in the next calendar year for pregnancies that begin later in the year.
If your LMP started on July 10, the estimated due date comes out to roughly April 16 of the following year, putting the third trimester (starting around week 28) in early January.
How to Use the Pregnancy Due Date Calculator
Step 1
Enter the first day of your last menstrual period.
Step 2
View your estimated due date, current week of pregnancy, and trimester guidance.
Step 3
Watch the progress bar update as the pregnancy advances toward 40 weeks.
Step 4
Re-check the estimate against an ultrasound dating scan at your first prenatal appointment.
Step 5
Use the current week shown to know which trimester-specific guidance applies to you right now.
Step 6
Revisit the calculator periodically to track how many weeks remain as your due date approaches.
Benefits
- Turns your due date estimate into a branded image card you can save or send to family right away.
- Gives an immediate estimate without needing an ultrasound.
- Shows current week of pregnancy and trimester at a glance.
- Useful starting reference to bring to your first prenatal appointment.
- Updates automatically each time you revisit, so "weeks pregnant" always reflects today's date.
- Free, private, and doesn't require creating an account to use.
Common Pregnancy Due Date Calculator Scenarios
Scenario 1
Getting an early estimate before your first doctor visit.
Scenario 2
Tracking which trimester you are currently in.
Scenario 3
Planning ahead for maternity leave or major life events around the due date.
Scenario 4
Deciding when to schedule the first prenatal appointment and dating ultrasound.
Scenario 5
Estimating a rough timeline for nursery preparation or baby shower planning.
Scenario 6
Comparing an LMP-based estimate against a date given by an ultrasound for context.
Understanding Your Result
The estimated due date marks the average expected delivery date for a 40-week pregnancy counted from your LMP. It is a statistical average, not a guarantee — full-term delivery is considered anywhere from 37 to 42 weeks.
The trimester guidance shown alongside your current week reflects three broad stages: the first trimester (through about week 13) is when prenatal vitamins, hydration, and early checkups matter most; the second trimester (roughly weeks 13-27) is often when energy levels improve; and the third trimester (week 27 onward) is focused on preparing for delivery and attending more frequent appointments. These boundaries are approximate guidance, not sharp clinical cutoffs.
Tips
- Confirm this estimate with an ultrasound dating scan, which is generally more accurate.
- Track how weeks progress rather than fixating on the exact due date.
- Bring this estimate to your first prenatal visit for comparison.
- Keep a note of your exact LMP date, since it's easy to forget the precise day once your pregnancy has progressed.
- Treat the due date as the middle of a range, not a fixed deadline, when planning leave or travel.
Common Mistakes
- Assuming the due date is exact — only about 5% of babies arrive on their calculated due date.
- Using an irregular or inaccurate LMP date, which throws off the entire estimate.
- Not accounting for cycles longer or shorter than 28 days, which shifts actual ovulation timing.
- Skipping the first prenatal ultrasound, which typically gives a more precise dating than LMP alone.
- Confusing "weeks pregnant" with the number of weeks since conception, which is usually about two weeks fewer.
Frequently Asked Questions
How accurate is this due date estimate?
Naegele's Rule provides a standard estimate, but only about 5% of babies are born exactly on their due date — actual delivery can occur anywhere from 37 to 42 weeks.
What if my cycle isn't 28 days?
This estimate assumes a standard 28-day cycle; if your cycle is significantly longer or shorter, an ultrasound-based estimate from your doctor will be more accurate.
When should I see a doctor?
Schedule your first prenatal appointment as soon as you know or suspect you're pregnant, regardless of this estimate.
Is due date calculated from LMP or conception?
Naegele's Rule calculates from the first day of your last menstrual period, not from conception, which typically occurs about 2 weeks later.
What counts as full term?
Full term is generally considered 39 to 40 weeks, with "early term" at 37-38 weeks and "late term" at 41 weeks — all within the normal range.
Can an ultrasound change my due date?
Yes, an early ultrasound (especially in the first trimester) is often considered more accurate than LMP-based dating and may adjust your due date.
Why does the estimate change if I re-enter the same LMP?
It shouldn't for the same LMP date — if the estimated due date stays fixed but 'weeks pregnant' changes, that's expected since it's calculated relative to today's date.
Does this work for irregular cycles?
Not reliably — Naegele's Rule assumes regular 28-day cycles, so irregular cycles make this estimate less accurate; a doctor-ordered ultrasound is recommended instead.
Do most babies arrive exactly on their due date?
No — only about 5% of babies are born on their exact due date. Any birth between 37 and 42 weeks is considered full term and normal timing.
Can I share my due date estimate as an image?
Yes — tap Share and, on supported devices, your estimated due date is shared as a branded image card, not just a text link.
Does the due date change for twins or multiples?
Naegele's Rule itself doesn't adjust for multiples, but pregnancies with twins or more often deliver earlier than 40 weeks on average, so your doctor will typically use a separate, more tailored timeline for monitoring.
What's the difference between gestational age and fetal age?
Gestational age is counted from the first day of your LMP (what this calculator uses), while fetal age is counted from the estimated date of conception, which is usually about two weeks later — so fetal age is typically about two weeks less than gestational age at any point.
Can this calculator be used after IVF?
Not reliably — IVF pregnancies are usually dated from a known egg retrieval or embryo transfer date rather than LMP, since the exact conception timing is already known and more precise than an LMP-based estimate.
How many weeks is considered each trimester?
The first trimester runs through about week 13, the second trimester from roughly week 13 to week 27, and the third trimester from week 27 until delivery — though sources vary slightly on the exact week boundaries.
References
Important Information
This tool is for educational purposes only. Not medical advice.
Last updated: July 25, 2026