Quick answer
Your due date is the first day of your last menstrual period plus 280 days (40 weeks), by Naegele's rule for a 28-day cycle; add the extra days if your cycle is longer. A last period starting 10 January 2026 gives a due date of 17 October 2026. From a known conception date, add 266 days. It's an estimate; your doctor or midwife will confirm it with an ultrasound.
How is the due date calculated?
Enter the first day of your last period (with your usual cycle length), a conception date, or an ultrasound result (scan date and the gestational age on the report). You get the estimated due date, how many weeks and days pregnant you are today, which trimester you're in, and the actual dates of key checks: the nuchal translucency scan, anatomy scan, glucose test and group B strep test.
How to work out your due date
- 1
Choose the method: last menstrual period, conception date or ultrasound result.
- 2
By period: pick the FIRST day of bleeding of your most recent period and your usual cycle length (28 days by default).
- 3
By ultrasound: enter the scan date and the gestational age on the report (for example 12 weeks 3 days). First-trimester scans are the most accurate.
- 4
Read the due date, today's gestational age, the days to go and the milestones; “Copy” saves or shares them.
Milestones by gestational age
| Gestational age | What usually happens | Example: last period 10 Jan 2026 |
|---|---|---|
| 6 – 12 weeks | First antenatal visit, dating the pregnancy | 21 Feb – 10 Apr 2026 |
| 11 – 13 weeks 6 days | Nuchal translucency scan, prenatal screening | 28 Mar – 17 Apr 2026 |
| 18 – 22 weeks | Anatomy (anomaly) scan | 16 May – 19 Jun 2026 |
| 24 – 28 weeks | Glucose tolerance test (gestational diabetes) | 27 Jun – 31 Jul 2026 |
| 36 – 37 weeks 6 days | Group B strep test | 19 Sep – 2 Oct 2026 |
| 39 – 40 weeks 6 days | Full term | 10 Oct – 23 Oct 2026 |
Week ranges follow the American College of Obstetricians and Gynecologists (ACOG) and WHO; your own schedule is set by your clinician and clinic.
Terms for gestational age at birth (ACOG)
| Gestational age | Term |
|---|---|
| Before 37 weeks | Preterm |
| 37 weeks – 38 weeks 6 days | Early term |
| 39 weeks – 40 weeks 6 days | Full term |
| 41 weeks – 41 weeks 6 days | Late term |
| 42 weeks or more | Post-term |
ACOG Committee Opinion 579, “Definition of Term Pregnancy” (2013, since reaffirmed).
Examples
A regular 28-day cycle
Last period started 10 January 2026: 10 Jan + 280 days = 17 October 2026. On 1 June 2026 you'd be 20 weeks 2 days, in the second trimester.
A 32-day cycle
Same last period, 10 January 2026, but a cycle 4 days longer than standard means ovulating about 4 days later: due date 21 October 2026.
Known conception date
Conception on 1 March 2026: 1 Mar + 266 days = 22 November 2026. For IVF, the conception date is the transfer date minus the embryo's age (5 days back for a day-5 embryo).
From a first-trimester scan
A scan on 15 May 2026 shows 12 weeks 3 days (87 days): 280 − 87 = 193 days to go, so the due date is 24 November 2026.
Common mistakes
- Entering the day your period ended, or the day you think you conceived, as the last period.
- Naegele's rule counts from the FIRST day of your last period, which is why gestational age runs about two weeks ahead of the embryo's real age.
- Treating the due date as the day the baby will arrive.
- Only about 4–5% of babies are born on their due date; anywhere from 37 to 42 weeks is normal.
- Changing the due date after every scan.
- Clinicians usually keep the date from the earliest reliable scan (first trimester); later scans track growth rather than re-date the pregnancy.
- Relying on the period method with very irregular cycles.
- If your cycles are irregular or you don't remember your last period, an ultrasound is the better way to date the pregnancy.
When does an ultrasound override the period date?
According to ACOG Committee Opinion 700, “Methods for Estimating the Due Date” (2017), measuring crown–rump length in the first trimester is the most accurate way to date a pregnancy. If the scan differs from the period date by more than a threshold (over 5 days for scans before 9 weeks, over 7 days for scans at 9 to 13 weeks 6 days), the due date is usually changed to match the scan.
The calculator lets you try both methods and compare; the final call belongs to the clinician looking after your pregnancy.
Please note
This is a reference tool, not a substitute for antenatal care. WHO guidance (2016) recommends a first contact within the first 12 weeks and at least eight antenatal contacts; book in early, and seek care straight away for bleeding, severe abdominal pain, swelling, headaches or reduced baby movements.
The dates you enter are calculated in your browser and never sent anywhere.
Glossary
- Naegele's rule
- Due date = first day of last period + 1 year − 3 months + 7 days, the same as adding 280 days.
- Gestational age
- Counted from the first day of the last period, in weeks and days (for example 20 weeks 2 days).
- Trimester
- The three stages of pregnancy: up to 13 weeks 6 days, 14 to 27 weeks 6 days, and 28 weeks onwards.
- Nuchal translucency (NT)
- Fluid at the back of the baby's neck measured by ultrasound at 11 to 13 weeks 6 days, used in chromosomal screening.
- Crown–rump length (CRL)
- A first-trimester ultrasound measurement used to date the pregnancy.
Frequently asked questions
How accurate is a due date from my last period?
Fairly close if your cycles are regular and you remember the date; it's usually within a few days to a week or two. A first-trimester scan is more accurate.
My cycle is 35 days. What do I do?
Set the cycle length to 35 days; the calculator adds 7 days compared with the standard 28-day cycle because you ovulate later.
Is it OK to give birth at 37 weeks?
From 37 weeks counts as early term. ACOG still advises against planned delivery before 39 weeks without a medical reason.
How far past the due date is a concern?
From 41 weeks clinicians usually monitor more closely; from 42 weeks the pregnancy is post-term and needs care as advised.
Does it work for IVF pregnancies?
Yes: choose “Conception” and enter the transfer date minus the embryo's age (3 days back for a day-3 embryo, 5 for a day-5 embryo).
Do you store the dates I enter?
No. Everything is calculated in your browser.
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