Finding out you are pregnant often brings one question straight away: when is the baby due? A due date helps you plan appointments, scans, and work, even though it is only an estimate.
Enter the first day of your last period and your usual cycle length, and this calculator works out your estimated due date, conception date, how far along you are, your trimester, and days left.
Important: This calculator gives an estimate only. It is not medical advice and does not replace care from a healthcare provider. Please confirm your dates with your provider, who may date the pregnancy with an ultrasound scan.
Pregnancy Due Date Calculator
Enter the first day of your last period and your usual cycle length to estimate your due date, how far along you are, and your current trimester. This is an estimate only and is not medical advice.
Your results
A due date is an estimate. Only about 4–5% of babies arrive on the exact due date. Please confirm your dates with your healthcare provider, who may use an ultrasound scan to date the pregnancy.
What Is the Pregnancy Due Date Calculator?
The Pregnancy Due Date Calculator is a free tool that estimates when a baby is likely to be born. It sits with our other tools in the health calculators collection.
Instead of counting on a calendar by hand, the tool does the date maths in seconds and adjusts for cycle length — which matters if your cycle is shorter or longer than 28 days.
How It Works and the Formula
The calculator uses Naegele’s rule, a long-standing method for estimating a due date from the last period.
Formula: Estimated Due Date (EDD) = LMP + 280 days + (cycle length − 28 days)
- LMP: The first day of your last menstrual period. This is day one of the count, even though conception usually happens about two weeks later. It is used because most people remember it more clearly than the conception date.
- 280 days: The typical length of pregnancy from the LMP — 40 weeks exactly, including the roughly two weeks before ovulation.
- Cycle length: Days from the first day of one period to the first day of the next, on average. The standard assumption is 28 days.
- Cycle adjustment: A longer cycle likely means later ovulation, so days are added; a shorter cycle means days are taken away. A 35-day cycle adds 7 days; a 24-day cycle subtracts 4.
- Conception estimate: Shown as EDD minus 266 days (38 weeks), the estimated time from conception to birth.
- Gestational age: The weeks and days between the LMP and the reference date, usually today.
How to Use the Pregnancy Due Date Calculator
- Enter the first day of your last menstrual period in the LMP field — the first day of bleeding, not the last.
- Enter your average cycle length in days. Use 28 if you are not sure, or average your last few cycles.
- Check the today / reference date. It defaults to today; change it only to check another date.
- Click Calculate and read your due date, conception estimate, gestational age, trimester, and days remaining.
- Confirm the dates with your healthcare provider at your first appointment, especially if a scan differs.
Worked Example
Suppose the first day of the last period was 1 January 2026 and the cycle length is 28 days.
Add 280 days to 1 January 2026. There is no adjustment, because 28 − 28 = 0. The estimated due date is 8 October 2026.
Now keep the same LMP but use a 35-day cycle. The adjustment is 35 − 28 = 7 days, so 7 days are added. The estimated due date becomes 15 October 2026.
The conception estimate for the 28-day example is the due date minus 266 days, in mid-January 2026 — about two weeks after the LMP. That gap is normal.
Pregnancy Trimesters at a Glance
Pregnancy is usually divided into three trimesters. This table is general information only.
| Trimester | Weeks | What is typically checked |
|---|---|---|
| First trimester | 0–13 weeks | Confirming the pregnancy and due date, first blood tests, and an early dating ultrasound where offered. |
| Second trimester | 14–27 weeks | An anatomy ultrasound, usually around 18–22 weeks, growth checks, and screening such as glucose testing later in this stage. |
| Third trimester | 28 weeks until birth | More frequent check-ups, growth and position checks, blood pressure monitoring, and planning for labour and birth. |
What is offered varies by provider, so follow your own provider’s schedule.
Important Points and Assumptions
- Only about 4–5% of babies arrive on the exact due date. Many healthy babies are born in the two weeks before or after it, so treat it as the centre of a window, not a deadline.
- The method assumes ovulation happened mid-cycle and cycles are fairly regular. Irregular cycles make period-based dating less reliable.
- Ultrasound dating can differ from the period date. Providers often use the scan date if the difference is large enough.
- This estimate uses only the dates you enter. It cannot account for your medical history or fertility treatment.
- For a related health check, our BMI calculator for checking body mass index can be useful when your provider advises it.
Common Mistakes
- Using the last day of the period instead of the first. The LMP is always the first day.
- Guessing cycle length from one month only. One unusual cycle can shift the answer; use an average where you can.
- Treating the due date as exact. Plan for a window of a few weeks, not a single day.
- Ignoring a different scan date. If your provider gives a revised due date after a scan, use that date for appointments.
- Entering dates the wrong way round. If you see an error that the LMP is after the reference date, check both dates.
How to Interpret Your Results
Your estimated due date is the main planning date. Use it to follow week-by-week guides and time appointments, but keep plans flexible near the end of pregnancy.
Your gestational age, in weeks plus days, is how providers describe timing — for example, 20 weeks 3 days means you are in your 21st week. Your trimester gives the broader stage, and days remaining counts to the due date.
If your provider discusses energy needs, our TDEE calculator for daily energy expenditure and the advanced calorie calculator for detailed calorie targets explain the general ideas — but in pregnancy, always follow your provider’s personal advice.
Frequently Asked Questions
How accurate is a due date calculator?
It is a good starting estimate when periods are regular and the LMP is known. It is less reliable with irregular cycles or an uncertain LMP. An early ultrasound is generally more accurate, and your provider may revise the date based on a scan.
Why is pregnancy counted from my last period and not from conception?
Because the first day of the last period is a date most people can identify, while the exact day of conception is hard to know. That is why a 40-week pregnancy includes about two weeks before conception.
What if my cycle is irregular?
Use your best average, but expect less certainty. Mention irregular cycles at your first appointment; your provider will likely rely more on an early ultrasound.
Can my due date change?
Yes. If an ultrasound, especially in the first trimester, suggests a noticeably different date, many providers update it. After that, the date is usually kept even if later scans show different measurements.
What does it mean if I go past my due date?
Many pregnancies continue a few days past the due date. Your provider will monitor you and the baby more closely and discuss next steps, including induction, if it continues well beyond it.
I do not know the date of my last period. What should I do?
Use your best estimate if you have one. Otherwise, a provider can date the pregnancy with an ultrasound and questions about your cycle and when a test first turned positive.
References
- American College of Obstetricians and Gynecologists (ACOG) — guidance on estimating the due date and methods for dating pregnancy. See https://www.acog.org/
For more tools, return to the health calculators hub and explore the full collection.