Answer: Enter your values and the Pregnancy Due Date Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Calculate your pregnancy due date based on your last menstrual period or conception date.
This due date calculator estimates a delivery date from the first day of your last menstrual period (LMP) or from a conception date, then shows your current gestational week, days remaining, and trimester. It uses Naegele's rule — the standard obstetric method of adding 280 days (40 weeks) to the LMP — and adjusts for cycles longer or shorter than the 28-day default.
Only about 4 in 100 babies arrive exactly on their estimated due date; the date is a midpoint, not an appointment. Roughly half of births happen within a week of it. Use this calculator for planning and milestone tracking, and rely on your clinician's dating — typically confirmed by first-trimester ultrasound, which is more accurate than calendar math — for medical decisions.
This page explains where the 280-day figure comes from, how cycle-length adjustment works, what each trimester covers, and answers the most common questions about due-date math.
Gestational age counts weeks from the LMP even though conception happens about two weeks later — so at conception you are officially 'week 2.' That convention keeps every pregnancy on one timeline and is why full term at 39–40 weeks corresponds to 37–38 weeks after conception. Hospital paperwork, screening windows, and leave planning all key off these week numbers.
If your cycles are irregular, recently changed, or you were breastfeeding when you conceived, LMP math degrades quickly — a cycle that varies by more than a few days shifts ovulation, and with it the whole 280-day count. In that case an early ultrasound measurement of the embryo gives a due date accurate to within roughly 5–7 days, which is why clinicians prefer it for dating.
Beyond the due date itself, the same timeline drives practical planning: parental leave typically starts around 39 weeks or at delivery, most airlines allow travel through week 36 with a note, and Group B strep screening is scheduled at 36–37 weeks. Knowing your week count, not just the due date, is what schedules those checkpoints.
One more nuance: 'due date' and '40 weeks' are the same day, but delivery anywhere from 37 to 42 weeks is considered normal term. First babies average a few days past 40 weeks, which surprises many expectant parents who planned around the exact date.
Naegele's rule: estimated due date = LMP + 280 days. The 280 days equal 40 weeks from LMP, which assumes ovulation and conception around day 14 of a 28-day cycle. If you enter a conception date instead, the tool subtracts 14 days to reconstruct an equivalent LMP and then applies the same rule — matching the clinical convention that conception-to-birth averages about 266 days.
Cycle adjustment: if your cycle is longer or shorter than 28 days, ovulation shifts by roughly the same amount. The calculator adds or subtracts the difference (cycle length − 28) from the 280-day span, so a 30-day cycle adds 2 days and a 26-day cycle subtracts 2.
Gestational age is counted from the LMP, not conception: week = whole days since LMP ÷ 7. Trimesters follow the clinical convention — first through completed week 12, second from week 13 through week 26, third from week 27 to delivery.
The table below shows the milestone dates the 280-day timeline produces for a sample LMP of January 1, 2026, and what each marks. Your own dates shift with your LMP, but the week numbers and intervals are the same for every pregnancy dated this way.
Two conversions are worth remembering: the first trimester ends at day 84 (week 12), the second at day 189 (week 27), and full term begins at 259 days (37 weeks), with 280 days marking 40 weeks — the estimated due date itself. Viability landmarks used in prenatal screening (nuchal translucency, anatomy scan, glucose testing) are scheduled against these week numbers.
| Milestone | Timing | Example (LMP Jan 1, 2026) |
|---|---|---|
| End of first trimester | Week 13 begins (day 84) | March 25, 2026 |
| End of second trimester | Week 27 begins (day 189) | July 8, 2026 |
| Early term | 37 weeks (day 259) | September 17, 2026 |
| Full term | 39–40 weeks (days 273–280) | October 1–8, 2026 |
| Estimated due date | 40 weeks (day 280) | October 8, 2026 |
LMP-based dating assumes a regular 28-day cycle with mid-cycle ovulation. Studies show first-trimester ultrasound (crown-rump length before ~13 weeks) dates a pregnancy more precisely, and clinicians re-date based on it when the two disagree by more than about a week.
The calculator is a planning aid, not a medical device. Irregular cycles, unknown LMP, breastfeeding, or recently stopping hormonal contraception all reduce LMP accuracy — in those cases enter a conception date if you know it, or wait for an ultrasound-based estimate.
How is my due date calculated from my last period?
By Naegele's rule: 280 days (40 weeks) are added to the first day of your last menstrual period. The method assumes ovulation around day 14 of a 28-day cycle, so conception actually occurred about two weeks after your LMP.
What if my cycle is not 28 days?
Ovulation shifts with cycle length, so the calculator adds or subtracts the difference from 28. A 32-day cycle moves your due date 4 days later; a 26-day cycle moves it 2 days earlier.
How accurate is a due date calculator?
LMP-based estimates have a spread of a couple of weeks either way because ovulation varies. First-trimester ultrasound dating is more accurate. Only about 4% of babies are born on the exact date — most arrive within about a week of it.
How do I calculate a due date from conception?
Average time from conception to birth is about 266 days. This calculator subtracts 14 days from your conception date to derive an LMP-equivalent and then applies the standard 280-day rule, giving the same result as adding 266 days directly.
Which trimester am I in?
First trimester runs from week 1 to the end of week 12, second trimester from week 13 to the end of week 26, and third trimester from week 27 until delivery. The calculator labels your trimester automatically from your current week.