Due Date Calculator
The first question after a positive test is usually 'when?'. This due date calculator turns the first day of your last period into an estimated due date using the standard 280-day rule, adjusted for your average cycle length, and shows how many weeks pregnant you are, your trimester, the days remaining and the likely conception date. It assumes a regular cycle. Treat the result as a solid estimate; your midwife or doctor will confirm the date with an ultrasound, the gold standard for dating a pregnancy.
Last updated: October 22, 2026
Your result
The formula
Due date = first day of last period + 280 daysCycle adjustment = + (cycle length − 28) daysWeeks pregnant = days since last period ÷ 7, rounded downTrimester = 1st (weeks 1 to 13), 2nd (weeks 14 to 27), 3rd (week 28+)Conception date ≈ last period + 14 days, adjusted for cycle length
A worked example
With a last period starting 1 January 2026 and a 28 day cycle, the due date is 8 October 2026, 280 days later. On 26 September 2026 that is 38 weeks pregnant, in the third trimester, with about 12 days to go, and the likely conception date was 15 January 2026.
More about the Due Date Calculator
How the 280-day rule actually works
Pregnancy is dated from the first day of your last period, not from conception, which is why a missed period already means about 4 weeks pregnant. Add 280 days (40 weeks) to that date and you have the estimated due date. With a last period starting 1 January 2026 and a 28 day cycle, the due date lands on 8 October 2026.
The cycle adjustment matters more than people think. The standard rule assumes ovulation on day 14 of a 28 day cycle; if your cycle is 35 days, ovulation, and the due date, shift about 7 days later. Enter your real average cycle length, not the textbook 28.
Why the date on the screen is not an appointment
Only about 5% of babies arrive on their due date; most come within two weeks either side. The date is a planning anchor, for booking leave, scheduling scans, buying the car seat, not a deadline your body signed.
If an early ultrasound disagrees with the period based date by more than a few days, care providers usually adopt the scan date. Direct measurement of the baby beats calendar math, which is why the first trimester scan is the dating scan.
What the other numbers mean
Weeks pregnant is counted from the last period and rounded down. It is the number your care provider will ask for at every appointment. The trimester follows the standard split: weeks 1 to 13, 14 to 27, then 28 onwards, each with its own rhythm of scans and milestones.
The conception estimate (roughly last period + 14 days, adjusted for your cycle) is a guide, not a fact, ovulation varies. It is useful context, not something to set your watch by.
When the calculator gets it wrong
Irregular cycles are the biggest source of error. If your cycles swing between 28 and 40 days, the average you enter is a guess and the due date inherits that uncertainty, mention the irregularity at your first appointment so the dating scan gets priority.
Recent hormonal contraception, breastfeeding or a very recent miscarriage can all scramble the 'last period' anchor. In those cases, treat the calculator's date as a rough placeholder and let the ultrasound do the real dating.
Planning around the date without stressing over it
Use the due date to work backwards: count back 12 weeks for the end of the first trimester, 4 weeks for when to start the hospital bag, 2 weeks for the 'she could come any day now' zone that relatives should respect. A date on the calendar turns vague anxiety into a checklist.
Tell work your due date but plan leave from 38 weeks, most providers consider 37 weeks full term, and babies rarely consult the calendar. Having the nursery ready by 36 weeks means the last month is for resting, not flat pack furniture.
Telling people the date (or not)
Many parents learn to quote a vaguer date, 'early October' instead of 'October 8th', because a specific date invites daily 'any news?!' messages from week 38 onward. The calculator gives you precision, but sharing it is optional.
Give your employer the real date (they need it for leave planning) and give relatives the month. This is not deception; it is boundary management during the most messaged about weeks of your life.
Due dates and induction decisions
As the due date passes without labor, providers may discuss induction, typically offered between 41 and 42 weeks. The calculator's date anchors these conversations; '41 weeks' means 7 days past the date above.
Ask for the specific medical reasons, the proposed method, and the alternatives (including waiting with monitoring). Induction is common and safe, but it is still your decision, informed consent means understanding the why, not just the when.
Bring the printed calculation to appointments. When everyone references the same date, discussions about timing stay grounded instead of drifting into approximations.
Due date math for IVF pregnancies
IVF pregnancies are dated from egg retrieval or embryo transfer, not LMP, day 14 of the 'pregnancy' is roughly the retrieval date. Add 266 days to the retrieval date (261 for a day 5 transfer) for the due date.
This makes IVF dates more accurate than LMP based ones, there is no ovulation guesswork. If your clinic gave you a date, trust theirs over any calculator; they know the exact transfer day.
For donor egg or frozen transfers, the same transfer date math applies. The biology of dating does not change, only the certainty of day zero improves.
How to use the Due Date Calculator
- Enter the first day of your last menstrual period, day one of bleeding, not spotting.
- Set your average cycle length; leave it at 28 days if your cycles are regular.
- Press Calculate to see the estimated due date, weeks pregnant and trimester.
- Treat the conception estimate as a guide only, an early ultrasound gives the most accurate dating.
Frequently asked questions
How is the due date calculated?
With the standard 280-day rule, 40 weeks added to the first day of your last period. The calculator also adjusts for your cycle length, since longer cycles push ovulation (and the due date) later by the same number of days.
How accurate is a due date calculator?
It is an estimate, only about 5% of babies arrive on the exact date, with most born within two weeks either side. A first trimester ultrasound measures the baby directly and usually gives a more reliable date.
Can the due date change later?
Yes, if an early ultrasound disagrees with the period based date by more than a few days, your care provider will usually adopt the scan date. Keep both dates in your notes until the dating scan settles it.
What does EDD mean?
Estimated date of delivery, another name for the due date. You will see EDD on scan reports and midwife notes once the pregnancy is dated.
How many weeks pregnant am I right now?
Count from the first day of your last period to today and round down. That is exactly what the 'weeks pregnant' row on this calculator does. Dating starts at the period, which is why a missed period already means about 4 weeks pregnant.
Is conception based dating more accurate than period based?
If you tracked ovulation, yes, the period method assumes a regular 28 day cycle with ovulation on day 14, which is wrong for many women. Conception dating skips that assumption entirely.
Is 39 weeks considered full term, or does full term start at 40?
Full term officially runs from 39 weeks through 40 weeks and 6 days, so a baby born at 39 weeks counts as full term. The due date marks 40 weeks exactly, which is the midpoint of full term rather than the finish line. Babies born before 37 weeks are considered preterm, and a pregnancy continuing past 42 weeks is called post term.
What trimester am I in and when do they change?
The first trimester runs from week 1 through week 13, the second from week 14 through week 27, and the third from week 28 until delivery at around week 40. Most pregnancy symptoms, screenings and milestones are tracked by these divisions, for example, the anatomy scan typically happens around week 20 in the second trimester. A due date calculator gives your current week, from which the trimester follows directly.
What if I don't remember my last period date?
If you don't remember your last period, an early ultrasound (weeks 7 to 12) is the most accurate way to date the pregnancy, with an error margin of a few days. Your doctor will confirm the due date with that measurement.
What is Naegele's rule?
It is the classic method to estimate the due date: add 7 days, subtract 3 months and add 1 year to the first day of the last period. It assumes a 28 day cycle with ovulation on day 14; an early ultrasound adjusts the date more precisely.