How the estimate works
This uses Naegele's rule, the standard method:
Estimated due date = First day of last period + 280 days (40 weeks)
Pregnancy is counted from the first day of your last period rather than from conception, which is why "four weeks pregnant" usually means about two weeks after conception. It's counted this way because the date of the last period is something you can usually pin down, while the date of conception rarely is.
If your cycle is longer or shorter than 28 days, the estimate shifts. A 32-day cycle typically pushes the due date about four days later.
Worked example
Last period began 1 March 2026, with a 28-day cycle:
| Milestone | Date |
|---|---|
| First day of last period | 1 March 2026 |
| Approximate conception | around 15 March 2026 |
| End of first trimester | around 24 May 2026 |
| End of second trimester | around 30 August 2026 |
| Estimated due date | 6 December 2026 |
Trimesters
| Trimester | Weeks | Roughly |
|---|---|---|
| First | 1 – 13 | Months 1–3 |
| Second | 14 – 27 | Months 4–6 |
| Third | 28 – 40+ | Months 7–9 |
A pregnancy is considered full term from 39 weeks. Anything from 37 weeks onward is generally regarded as term, and 42 weeks is usually the point at which doctors discuss induction.
How accurate is a due date?
Around 4–5% of babies arrive on the estimated date. Most births happen within two weeks either side, and first pregnancies tend to run slightly longer.
An early ultrasound, particularly a dating scan in the first trimester, is more accurate than a date calculated from your last period, especially if your cycles are irregular or you are unsure of the date. Where the two disagree by more than about a week, doctors usually go with the scan.
Contact your doctor promptly if you have bleeding, severe or persistent pain, a sudden reduction in your baby's movements, severe headache with vision changes, or a fever. Do not wait for your next appointment.
Frequently Asked Questions
How is a due date calculated?
By adding 280 days, or 40 weeks, to the first day of your last menstrual period. This is known as Naegele's rule.
Why is pregnancy counted from my last period?
Because that date is usually known, while the exact date of conception rarely is. It means the first two weeks of the count happen before conception.
How accurate is the due date?
Only about 4–5% of babies are born on the estimated date. Most arrive within two weeks either side, and first pregnancies often run a little longer.
What if my cycle is not 28 days?
The estimate shifts. Longer cycles push the due date later, shorter cycles earlier. Your doctor will adjust for this, and an early scan is more reliable.
Is an ultrasound more accurate than this calculation?
Yes, especially a first-trimester dating scan. Where the scan and the period-based date differ by more than about a week, doctors generally use the scan.
When is a pregnancy considered full term?
From 39 weeks, though 37 weeks onward is generally regarded as term. Doctors typically discuss induction around 41–42 weeks.
Can this calculator confirm my pregnancy?
No. It only estimates dates. Confirmation and ongoing care come from your doctor or midwife.
Har trimester mein kya hota hai
| Trimester | Hafte | Aam jaanch |
|---|---|---|
| Pehla | 1–13 | Pushti, dating scan, khoon ki jaanch, iron aur folic acid shuru |
| Doosra | 14–27 | Anomaly scan (18–22 hafte), sugar ki jaanch, TT tika |
| Teesra | 28–40 | Growth scan, position dekhna, delivery ki taiyari |
Ye ek aam dhaancha hai. Aapke doctor aapki sehat ke hisaab se isme badlav kar sakte hain — unki salah hi maanein.
Doctor se turant milna kab zaroori hai
In mein se kuch bhi ho to usi din doctor se sampark kariye:
- Khoon aana ya pet mein tez dard
- Lagataar tez sar dard, dhundhla dikhna, ya achanak chehre aur haathon par soojan
- Tez bukhaar
- Bachche ki harkat kam ho jaana
- Paani ka rissav
Ye calculator sirf ek anuman deta hai aur kisi bhi tarah se doctor ki salah ka vikalp nahi hai. Har garbhavastha alag hoti hai — apni tareekhein, sehat aur koi bhi lakshan apne doctor se hi discuss kariye.
More questions
Anomaly scan kab hota hai?
Aam taur par 18 se 22 hafte ke beech. Aapke doctor aapki sthiti ke hisaab se samay bata denge.
Kam se kam kitni baar jaanch karwani chahiye?
Sarkari salah kam se kam chaar antenatal visits ki hai, aur pehle trimester mein registration karwane ki. Doctor zaroorat ke hisaab se zyada bhi bula sakte hain.
Related calculators and guides
Health disclaimer. This calculator provides general estimates for healthy adults and is not medical advice. It cannot account for your individual health conditions, medications or circumstances. Please speak to a doctor or a registered dietitian before making significant changes to your diet, exercise or fluid intake — particularly if you have any medical condition, are pregnant or breastfeeding, or have any history of disordered eating.