Three ways to calculate
The same baby has the same due date no matter which method you use, but you may not have the same starting information available. The calculator gives you three input methods so whatever you remember best becomes the input.
Last menstrual period is what most OB/GYN offices use. Pregnancy is conventionally counted from day one of the last period, even though conception happens about two weeks later. The standard 280-day pregnancy length is calculated this way. If your cycles are longer or shorter than 28 days, the calculator adjusts the due date accordingly, a 32-day cycle pushes the due date out by 4 days, a 24-day cycle pulls it in by 4 days.
Conception date is for people who know exactly when they conceived (basal temperature tracking, ovulation tests, IUI procedure dates). Adds 266 days to that date.
IVF transfer date handles the unique case of in-vitro fertilization. The math depends on whether the embryo was transferred on day 3 or day 5 after fertilization. Day-5 transfers are more common in modern IVF cycles. The calculator subtracts the right number of days so the calendar math works out the same as a natural conception due date.
Why the result is “estimated”
Only about 5% of babies actually arrive on their predicted due date. Roughly 80% are born within ten days on either side. About 10% arrive before 37 weeks (preterm); about 10% arrive after 41 weeks (post-term, when most providers will start discussing induction).
The 280-day standard is an average derived from population data. Real pregnancies vary based on maternal age, parity (whether this is a first baby), and individual genetics. First-time mothers tend to deliver slightly later than repeat mothers, on average by 3-5 days.
The most accurate due date estimate isn’t from a calculator, it’s from a first-trimester ultrasound. The crown-rump length measurement in weeks 7-13 is accurate to within 5 days. By the third trimester, ultrasound dating loses accuracy as babies grow at increasingly different rates.
Trimesters explained
The calculator shows which trimester you’re currently in. The cutoffs:
- First trimester: weeks 0-12 (some sources use 13). The most critical period for organ development. Miscarriage risk is highest here.
- Second trimester: weeks 13-26. Often called the “easier” trimester, energy returns, morning sickness usually fades. The anatomy ultrasound happens around week 20.
- Third trimester: weeks 27 to delivery. Rapid fetal growth, increasing maternal discomfort, prep for birth.
A “weeks plus days” notation is standard in obstetrics, 24w 3d means 24 weeks plus 3 additional days. Charts and growth curves are calibrated this way.
When the cycle-length adjustment matters
The default 28-day cycle assumption is the source of most calculator errors. If you have a 32-day cycle and use a standard “due date calculator” that ignores cycle length, your real due date will be about 4 days later than what the simple calculator says. With irregular cycles or unusually long/short cycles, the LMP method can be unreliable enough that providers prefer ultrasound dating.
If your cycles are very irregular (vary by more than 7 days month-to-month), use the conception date method if you can pinpoint it, or accept that the LMP calculation is a rough first guess and rely on ultrasound dating once that becomes available.
Frequently asked questions
My calculator says I’m 5 weeks pregnant but I just took a positive test, how? Pregnancy weeks count from the first day of your last period, not from conception. So you’re 5 weeks since your period started, but only about 3 weeks since the egg was fertilized. The math is just convention.
Can I move my due date to a more convenient calendar date? The due date is an estimate, not a target. Babies come when they’re ready. Induction for non-medical reasons before 39 weeks is generally discouraged.
What if I conceived during a contraception failure on a known date? Use the “conception date” method, that gives the most accurate result for known fertilization timing.
Why is the IVF calculation different? With IVF, the conception date is precisely known (the day eggs and sperm were combined in the lab), and the embryo’s age at transfer is exact. So the due date math runs forward from transfer minus the embryo’s age, which is more accurate than any LMP-based estimate.