Pregnancy Due Date Calculator, Explained
After reading this you can estimate a due date three different ways (from a last period, a conception date, or an IVF transfer), read off how many weeks along you are today, and know how wide the real spread around that date actually is.
What the calculator does and one quick example
A due date is a single calendar day at which a pregnancy reaches about 40 weeks counted from the last menstrual period (LMP). The word "estimated" carries most of the weight. The number is a midpoint, not a deadline.
Here is the core case. If your last period started on 2026-05-01 and your cycle runs 28 days, the standard estimate adds 280 days. That lands on 2027-02-04. That single date is what most people call the due date, and it is what the tool prints first.
The same tool also reports how far along you are today (gestational age) and a table of milestone dates, so you can see the estimate as a timeline rather than one lonely day.
When to use each counting method
The four methods differ only in what event they count from. They all aim at the same 40-week target measured from the LMP, so they convert to a common reference before adding days.
- Last menstrual period (LMP)
- Count from the first day of the last period. This is the traditional method and the default. It assumes ovulation near day 14.
- Conception date
- Count from a known conception or ovulation date. Conception happens roughly 14 days after the LMP, so the due date is conception plus 266 days.
- IVF day-3 transfer
- The embryo is already 3 days old at transfer. Due date is the transfer date plus 263 days.
- IVF day-5 transfer
- The embryo is 5 days old at transfer. Due date is the transfer date plus 261 days.
Do not use the LMP method if you do not know the true first day of your last period, or if your cycles are irregular. A guessed LMP can shift the estimate by a week or more. If you tracked ovulation or had an IVF transfer, use that method instead: it removes the guess about when you ovulated.
The formula and why the numbers are what they are
Naegele's rule sets the LMP-based estimate:
Here LMP is the first day of the last period, 280 is the standard length of a pregnancy in days (40 weeks), and \text{cycle} - 28 shifts the estimate when your cycle is not 28 days. A longer cycle means you ovulate later, so the due date moves later by the same number of days.
The other methods drop the parts of that 280 days that have already passed:
So conception plus 266 days equals the same target. A day-3 embryo has used 3 of the 266, giving 263. A day-5 embryo has used 5, giving 261. Each method reaches the identical due date when the inputs agree.
The cycle adjustment only applies to the LMP method. Conception and IVF methods already fix the moment of fertilization, so cycle length adds no information. The tool ignores the cycle field for those three methods.
A worked example you can reproduce
LMP on 2026-05-01, 28-day cycle
These are the demo defaults. Press the demo button, then Calculate, and you should see the same numbers.
- Start at the LMP:
2026-05-01. - Cycle adjustment: 28 - 28 = 0 days.
- Add 280 + 0 = 280 days.
- May 2026 has 30 days, so 280 days from May 1 lands on
2027-02-04.
Now change only the cycle to 32 days. The adjustment becomes 32 - 28 = 4 days, you add 284 days, and the due date moves to 2027-02-08. A four-day-longer cycle pushed the estimate four days later, exactly as the formula predicts.
To check the conception method, note that ovulation for a 28-day cycle falls near 2026-05-15 (LMP + 14). Counting 266 days from May 15 also lands on 2027-02-04. The two methods agree, as they should.
Reading gestational age and the milestone table
Gestational age is measured from the LMP, not from conception, which is why a pregnancy is described as "40 weeks" even though fertilization happened about 2 weeks in. On any given day, gestational age in weeks is the days since LMP divided by 7.
Suppose today is 2026-08-01 with the demo LMP of 2026-05-01. That is 92 days, or 92 \div 7 \approx 13.1 weeks: you would read this as 13 weeks and 1 day. The milestone table converts standard week counts into calendar dates using the same LMP anchor.
| Milestone | Weeks from LMP | Date |
|---|---|---|
| End of first trimester | 13 | 2026-07-31 |
| End of second trimester | 27 | 2026-11-06 |
| Considered full term | 37 | 2027-01-15 |
| Due date (40 weeks) | 40 | 2027-02-04 |
| End of "late term" | 42 | 2027-02-18 |
How wide the spread really is
The due date is one day, but births scatter around it. Only about 4 to 5 % of babies arrive on the estimated date itself. Most arrive within two weeks on either side, which is why full term covers 37 to 42 weeks rather than a single point.
The chart below sketches that spread as a rough distribution of birth timing relative to the due date. The peak sits near the due date, but the curve is wide and slightly skewed toward earlier delivery.
The practical reading: treat the due date as the center of a two-week window, not a promise. A first-trimester ultrasound gives the most reliable dating, because it measures the embryo directly rather than inferring from the LMP.
Try the cycle adjustment yourself
The single idea worth playing with is how cycle length slides the LMP-based estimate. Move the cycle and watch the due date shift day for day.
Common mistakes
A few errors change the answer by days or weeks.
- Counting from the wrong day. The LMP is the first day of bleeding, not the last. Using the end of the period shifts the estimate several days early.
- Applying the cycle adjustment to a conception or IVF date. Those dates already pin ovulation, so no cycle correction belongs there.
- Confusing gestational age with conception age. Gestational weeks run about 2 weeks ahead of time since conception. "8 weeks pregnant" means 8 weeks from the LMP, roughly 6 weeks from conception.
- Treating the date as fixed. An ultrasound estimate may differ from the LMP estimate by a week. Clinicians usually adopt the ultrasound date when the two disagree by more than a set threshold.
Related tools
Other everyday-health estimators on this site follow the same honest-about-error-bars approach. The Blood Alcohol Estimator uses the Widmark equation to estimate BAC and hours until sober. The Dose & Rate Converter handles mg/kg doses, mg-to-mL volumes, and infusion rates. Each gives a number with its limits stated plainly, not a guarantee.
Frequently asked questions
Why is pregnancy counted as 40 weeks when it lasts about 38?
Counting starts at the last period, roughly 2 weeks before conception. The 40-week figure includes those 2 weeks. Time from conception to birth is about 38 weeks, or 266 days.
What if my cycle is not 28 days?
Set your average cycle length. The LMP estimate shifts by \text{cycle} - 28 days. A 32-day cycle adds 4 days; a 25-day cycle subtracts 3.
Which method is most accurate?
An IVF transfer date is the most precise input, because the embryo's age is known exactly. A tracked conception date is next. LMP is the least certain, since it assumes ovulation on day 14. A first-trimester ultrasound outperforms all of them.
Can the calculator tell me the sex or health of the baby?
No. It only does date arithmetic. It produces an estimated timeline and nothing about the pregnancy's outcome or health.
My baby did not arrive on the due date. Was the estimate wrong?
Probably not. Only 4 to 5 % of births happen on the estimated date. Arriving within two weeks either side is the normal pattern, not an error in the math.
This is an educational estimate, not medical advice. Consult a professional for decisions about your health.
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