🤰 Pregnancy Due Date Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Medical Review, maternal health content review · Updated 2026-06-19
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.
This pregnancy due date calculator estimates when your baby is likely to arrive using the Naegele rule, the same method clinicians use. Enter the first day of your last menstrual period (LMP) and, if you know it, your average cycle length. You will instantly see your estimated due date, how far along you are today in weeks and days, the trimester you are in, and an approximate conception date.
What is the Pregnancy Due Date Calculator?
A due date is an estimate of 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 most people know the date their last period started, while the exact moment of conception is rarely known. This means the first two weeks of a counted pregnancy actually happen before you conceive, which is why gestational age is 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 accordingly, so this calculator adjusts the due date by the difference between your cycle length and 28 days. A 32-day cycle, for example, pushes the estimate about four days later, while a 24-day cycle brings it about four days earlier. This is a simple and widely used correction, sometimes called the Parikh adjustment in spirit, though individual cycles still vary.
Pregnancy is divided into three trimesters. The first 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. A pregnancy is considered full term from 37 weeks, and most births happen between 37 and 42 weeks. Remember that only a small share of babies actually arrive on the calculated date itself, so treat the result as the centre of a likely window rather than a fixed appointment.
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.
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.
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
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, usually most comfortable |
| Third trimester | Week 28 to birth | Rapid growth, full term from week 37 |
How cycle length shifts the estimated due date
| Average cycle | Adjustment to due date |
|---|---|
| 24 days | About 4 days earlier |
| 28 days (standard) | No change |
| 32 days | About 4 days later |
| 35 days | About 7 days later |
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.
Glossary
- LMP
- Last menstrual period, the first day of your most recent period. Pregnancy and the due date are counted from this date.
- 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
- A pregnancy that has reached 37 weeks of gestation, when the baby is considered ready for birth.
- Dating scan
- An early ultrasound that measures the baby to estimate gestational age, usually more accurate than an LMP estimate.
Frequently asked questions
How is the due date calculated?
It uses the Naegele rule: add 280 days (40 weeks) to the first day of your last menstrual period. This calculator 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 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.
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. A pregnancy is considered full term from 37 weeks, and most births happen between 37 and 42 weeks.
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) , American College of Obstetricians and Gynecologists