Pregnancy Calculator
This free pregnancy calculator estimates your due date, current gestational age, trimester, and key milestone dates, from your last period, an ultrasound, a known conception date, an IVF transfer date, or a due date your doctor already gave you. Nothing you enter is sent anywhere or stored.
How to Use It
- Choose how you want to calculate: Last Period, Due Date, Ultrasound, Conception, or IVF Transfer.
- Enter the date (and any other details) that method needs.
- Your estimated due date, current gestational age, trimester, and key milestones appear instantly.
How It Works
All five methods reduce to the same underlying date math, standard in obstetric care: a due date is estimated as 280 days from the first day of the last menstrual period (Naegele's rule), adjusted for cycle length if it isn't the typical 28 days. Conception-based methods use 266 days instead, since conception itself typically occurs about two weeks into a 28-day cycle. An IVF transfer date is adjusted further for the embryo's actual age (day 3, 5, or 6) at the time of transfer, since it wasn't "conceived" on transfer day.
Formulas
due date (from last period) = last period start + 280 days + (average cycle length − 28)
due date (from conception) = conception date + 266 days
Once a due date is established, every other figure (current gestational age, trimester, estimated conception date, and the first/third trimester and full-term milestone dates) is calculated the same way regardless of which method produced that due date.
A worked example using the Last Period method: a last period starting March 1, 2026 with a standard 28-day cycle gives an estimated due date of December 6, 2026 (280 days later). Calculated as of July 27, 2026, that pregnancy would be at 21 weeks and 1 day of gestational age, placing it in the second trimester (14 through 27 weeks).
Everything runs client-side in your browser. No dates you enter are sent anywhere or stored.
Frequently Asked Questions
Is the due date a prediction of when the baby will be born?
No, it's a schedule estimate based on standard obstetric dating conventions, not a prediction. Fewer than 4% of births happen on the exact estimated due date; most happen within a couple of weeks of it. Think of it as a planning reference, not a forecast.
Why do the different calculation methods give slightly different estimates?
Each method anchors to a different known reference point, and some are inherently more precise than others. An early ultrasound is generally considered the most accurate way to date a pregnancy, since it directly measures fetal size rather than relying on cycle regularity (as the last-period method does) or an assumed embryo age (as the IVF method does). If your doctor has given you a due date from an ultrasound, that should take precedence over any calculator.
Does this cover anything about my pregnancy besides dates?
No, this tool sticks to date-based scheduling: your due date, current gestational age, trimester, and key calendar milestones. It doesn't cover symptoms, fetal development, weight gain, or any other medical guidance. Talk to your doctor or midwife for anything beyond an estimated schedule.
Why is pregnancy dated from the last period instead of conception?
Historically, the last menstrual period (LMP) is a date most people can identify with reasonable confidence, while the exact moment of conception usually isn't directly observable. Standard obstetric dating (Naegele's rule) uses LMP as the anchor for exactly this reason, even though conception itself typically happens about two weeks into the cycle, which is also why gestational age is conventionally about two weeks ahead of actual embryonic/fetal age.
How is the trimester determined?
By current gestational age in weeks: the first trimester runs through week 13, the second trimester from week 14 through week 27, and the third trimester from week 28 onward. This calculator applies those standard cutoffs to whatever gestational age results from your chosen calculation method.
What counts as "full term"?
Full term is conventionally defined as 37 to 42 weeks of gestational age, measured from the last menstrual period. This calculator shows both the start (37 weeks) and end (42 weeks) of that window as milestone dates, alongside the estimated due date itself, which typically falls near the middle of that range at 40 weeks.
Can my due date change later in pregnancy?
Yes, if a later ultrasound measurement disagrees meaningfully with an earlier estimate, a doctor may adjust the due date, since a first-trimester ultrasound is generally considered the most accurate dating method. If your due date has been updated by a doctor, recalculating here using that updated date (or updated ultrasound measurement) will keep every other figure in sync with it.