🤰 Pregnancy Due Date Calculator: Estimated Due Date, Week and Trimester
By Shihab Mia · Reviewed by ToolNimba Medical Review, maternal health content review · Updated 2026-07-11
This calculator gives an estimate only and is not a diagnosis or medical advice. A due date based on your last period assumes a regular cycle and ovulation around day 14, which is not true for everyone. Only about 1 in 20 babies arrive on the estimated date, and an early ultrasound dating scan is more accurate. Always confirm your dates and any concerns with your doctor or midwife.
Your estimated due date is the first day of your last menstrual period (LMP) plus 280 days, or 40 weeks. Enter that date and your average cycle length above and this due date calculator instantly returns your estimated due date, how many weeks and days pregnant you are today, your current trimester and an approximate conception date. It uses the Naegele rule, the same 280-day method clinicians use, and adjusts for a cycle that is longer or shorter than the standard 28 days. Remember this is the centre of a likely window: most babies arrive within two weeks either side, and only about 5 percent are born on the exact date.
What is the Pregnancy Due Date Calculator?
A due date, or estimated date of delivery (EDD), marks when a full-term baby is most likely to be born. The standard method is the Naegele rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. Pregnancy is counted from the LMP rather than from conception because almost everyone knows when their last period started, while the exact moment of fertilisation is rarely known. A useful shortcut is to take the first day of your LMP, subtract three months and add seven days and one year. Counting from the LMP is also why the first two weeks of a pregnancy technically happen before you conceive, and why gestational age runs roughly two weeks ahead of the time since fertilisation.
The 280-day figure assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter, ovulation shifts and so does your due date, so this calculator adds or subtracts the difference between your cycle length and 28 days. A 32-day cycle pushes the estimate about four days later; a 24-day cycle brings it about four days earlier. This correction helps people with predictable but non-standard cycles, though it cannot fix genuinely irregular cycles, where an early scan is the reliable option.
The LMP is not the only way to date a pregnancy, and clinics choose the method that fits your situation. If you know your conception date, the EDD is that date plus 266 days. With IVF the timing is known precisely: a Day 5 blastocyst transfer adds 261 days to the transfer date and a Day 3 embryo transfer adds 263 days, which is why IVF due dates are among the most accurate. An ultrasound dates the pregnancy from the size of the embryo or fetus rather than from any recalled date.
According to the American College of Obstetricians and Gynecologists (Committee Opinion 700, 2017), a first-trimester ultrasound performed up to 13 weeks 6 days is the most accurate way to establish or confirm gestational age, dating a pregnancy to within about 5 to 7 days. Early embryos grow at nearly the same rate for everyone, which is what makes early scans so precise. If an early scan differs from your LMP date by more than 7 days, guidelines say the due date should be updated to match the scan. Accuracy falls later in pregnancy, to roughly plus or minus 10 to 14 days in the second trimester and 21 to 30 days in the third.
Pregnancy is divided into three trimesters: the first from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. In 2013 ACOG refined what counts as full term, because outcomes differ across the weeks around the due date. Early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days and postterm is 42 weeks and beyond. Treat your calculated date as a target at the centre of this range, not a fixed appointment on the calendar.
Use the result to plan appointments, maternity leave and time off, and to sanity-check a date you were given at a clinic. It does not replace prenatal care. Your dates, your health and the baby should always be confirmed and monitored by a qualified doctor or midwife, especially if your cycles are irregular, you conceived through fertility treatment, or you have any concerns.
When to use it
- Finding out roughly when your baby is due after a positive pregnancy test.
- Working out how many weeks pregnant you are today and which trimester you are in.
- Planning maternity leave, appointments and time off around an estimated arrival window.
- Cross-checking a date given at a clinic, or estimating before your first dating scan.
- Comparing an LMP estimate with a conception-date or IVF transfer date so you know why they differ.
- Timing pregnancy milestones such as the anatomy scan (around 20 weeks) or the full-term mark at 39 weeks.
How to use the Pregnancy Due Date Calculator
- Enter the first day of your last menstrual period (LMP), not the day it ended.
- Set your average cycle length in days, or leave it at 28 if you are unsure.
- Read your estimated due date, calculated as LMP plus 280 days adjusted for cycle length.
- Check your current gestational age in weeks and days, your trimester and the approximate conception date.
- If you have had an early dating scan, trust that date over the LMP estimate when they disagree by more than a week.
Formula & method
Worked examples
Your last period started on 1 January 2026 and you have a regular 28-day cycle.
- Cycle adjustment = 28 - 28 = 0 days
- Add 280 days to the LMP: 1 January 2026 + 280 days
- That lands on 8 October 2026
- No adjustment is needed for a 28-day cycle
Result: Estimated due date: 8 October 2026
Your last period started on 15 March 2025 and your cycles run about 32 days.
- Cycle adjustment = 32 - 28 = +4 days
- Base due date = 15 March 2025 + 280 days = 20 December 2025
- Apply the cycle adjustment: 20 December 2025 + 4 days
- That gives 24 December 2025
Result: Estimated due date: 24 December 2025
You had a Day 5 blastocyst transfer on 1 February 2026 through IVF.
- IVF uses the known transfer date, so cycle length does not matter
- For a Day 5 blastocyst, add 261 days to the transfer date
- 1 February 2026 + 261 days
- That lands on 20 October 2026 (a Day 3 transfer would add 263 days, giving 22 October 2026)
Result: Estimated due date: 20 October 2026
Pregnancy trimesters by gestational age (weeks from LMP)
| Trimester | Weeks | What it covers |
|---|---|---|
| First trimester | Week 0 to 13 | Conception, organ development, highest miscarriage risk |
| Second trimester | Week 14 to 27 | Movement felt, anatomy scan around week 20, usually most comfortable |
| Third trimester | Week 28 to birth | Rapid growth, full term from week 39 |
ACOG term definitions (Committee Opinion 579, 2013)
| Label | Gestational age | Note |
|---|---|---|
| Preterm | Before 37 weeks 0 days | Higher risk of complications |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Lungs and brain still maturing |
| Full term | 39 weeks 0 days to 40 weeks 6 days | Best window for planned delivery |
| Late term | 41 weeks 0 days to 41 weeks 6 days | Monitored more closely |
| Postterm | 42 weeks 0 days and beyond | Doctor may discuss induction |
How the dating method changes the calculation
| Method | What you enter | How the due date is found |
|---|---|---|
| LMP (Naegele rule) | First day of last period | Add 280 days, adjust for cycle length |
| Conception date | Known conception day | Add 266 days |
| IVF Day 5 transfer | Embryo transfer date | Add 261 days |
| IVF Day 3 transfer | Embryo transfer date | Add 263 days |
| Early ultrasound | Crown-rump length | Dated by scan, accurate to 5 to 7 days |
Common mistakes to avoid
- Using the wrong start date. The calculation needs the first day of your last period, not the day it finished or the day you think you conceived. Using the end date will push the estimate too late by several days.
- Treating the due date as a deadline. Only around 1 in 20 babies are born on their estimated due date. Most arrive within the two weeks either side, so think of it as the middle of a likely window rather than a fixed day.
- Ignoring an irregular cycle. The 280-day rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are long, short or irregular, an LMP estimate can be off by days or more, and an early dating scan will be more reliable.
- Confusing gestational age with time since conception. Gestational age is counted from the LMP, so it is about two weeks more than the time since fertilisation. Being told you are 8 weeks pregnant does not mean the embryo is 8 weeks old.
- Applying the LMP method to an IVF pregnancy. With IVF the conception date is known exactly, so the LMP shortcut is not needed. Use the transfer date instead: add 261 days for a Day 5 blastocyst or 263 days for a Day 3 embryo.
- Ignoring the scan when it disagrees with your dates. If a first-trimester ultrasound differs from your LMP date by more than 7 days, guidelines say the due date should be updated to the scan. Sticking with the old LMP date can leave you a week off.
Glossary
- LMP
- Last menstrual period, the first day of your most recent period. Pregnancy and the due date are counted from this date.
- EDD
- Estimated date of delivery, the calculated day your baby is most likely to be born, usually 40 weeks after the LMP.
- Naegele rule
- The standard method for estimating a due date by adding 280 days (40 weeks) to the first day of the last menstrual period.
- Gestational age
- How far along a pregnancy is, measured in weeks and days from the LMP, not from conception.
- Trimester
- One of the three roughly equal stages of pregnancy: weeks 0 to 13, 14 to 27, and 28 to birth.
- Full term
- Under the ACOG 2013 definition, a pregnancy of 39 weeks 0 days to 40 weeks 6 days, the best window for a planned birth.
- Dating scan
- An early ultrasound that measures the baby to estimate gestational age, usually more accurate than an LMP estimate.
- Crown-rump length
- The measurement from the top of the head to the bottom of the torso used by a first-trimester scan to date the pregnancy.
Frequently asked questions
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last menstrual period. This calculator uses that Naegele rule and also adjusts for your cycle length, adding or subtracting the difference between your cycle and a standard 28 days, since that changes when you ovulate.
How many weeks pregnant am I?
Count the days from the first day of your last period to today, then divide by 7. The calculator does this for you and shows the result as weeks and days, for example 12 weeks and 3 days. Because it is counted from the LMP, this gestational age is about two weeks more than the time since conception.
How accurate is a due date from my last period?
An LMP estimate is a reasonable guide for people with regular 28-day cycles, but it assumes ovulation on day 14, which is not true for everyone. Only about 1 in 20 babies arrive on the exact date, and most are born within two weeks either side. An early ultrasound dating scan is more accurate.
What if my cycle is not 28 days?
Cycle length changes when you ovulate, so it shifts the due date. This calculator adjusts by the difference between your cycle and 28 days. A 32-day cycle moves the estimate about four days later, and a 24-day cycle about four days earlier. For irregular cycles, a dating scan is the most reliable method.
How do I calculate a due date for IVF?
With IVF the conception date is known, so you count from the embryo transfer, not the LMP. Add 261 days for a Day 5 blastocyst transfer or 263 days for a Day 3 embryo transfer. Because the timing is exact, IVF due dates are among the most accurate estimates available.
Why is pregnancy counted from my last period and not conception?
Most people know when their last period started but not the exact day of conception, which usually happens about two weeks later. Counting from the LMP gives a consistent reference, which is why gestational age is roughly two weeks ahead of the time since fertilisation.
What are the trimesters?
The first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Under the ACOG 2013 definition, a pregnancy is full term from 39 weeks, and most births happen between 37 and 42 weeks.
Can my due date change?
Yes. If a first-trimester ultrasound (up to 13 weeks 6 days) differs from your LMP date by more than 7 days, ACOG guidance says the due date should be updated to the scan. In the second trimester the threshold is more than 14 days, and in the third trimester more than 21 days. The date is only reconciled after the first scan of the pregnancy.
What is considered overdue?
A pregnancy is full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks, and postterm at 42 weeks and beyond. Being a few days past your date is normal. If you pass 41 or 42 weeks your doctor will monitor you more closely and may discuss inducing labour.
Can I rely on this calculator instead of seeing a doctor?
No. This tool is an estimate to help you plan, not medical advice or a diagnosis. Your dates, health and the baby should always be confirmed and monitored by a qualified doctor or midwife, especially if your cycles are irregular or you have any concerns.
Sources
- Calculating a Due Date , Johns Hopkins Medicine
- Methods for Estimating the Due Date (Committee Opinion 700) , American College of Obstetricians and Gynecologists
- Definition of Term Pregnancy (Committee Opinion 579) , American College of Obstetricians and Gynecologists