One of the first questions after a positive pregnancy test is: when is my baby due? This guide explains how to calculate your pregnancy due date from the first day of your last period using Naegele's rule, how to adjust for longer or shorter cycles and IVF, how to calculate how many weeks pregnant you are, and what each trimester means – with worked examples.
What Is a Due Date (EDD)?
Your estimated due date (EDD) is the date when you will be 40 weeks pregnant. Doctors count pregnancy from the first day of your last menstrual period (LMP), not from conception, because the LMP is a date most people know and conception usually happens about two weeks later. So when you are "4 weeks pregnant", the embryo is only about 2 weeks old.
A full pregnancy is therefore 280 days (40 weeks) from the LMP, or about 266 days from conception.
Naegele's Rule: How to Calculate Your Due Date
The standard due date formula, called Naegele's rule, is:
Due date = first day of LMP + 280 days
An easy way to do it by hand:
LMP + 1 year − 3 months + 7 days
Worked example
If your last period started on 1 January 2026:
- Add 1 year → 1 January 2027
- Subtract 3 months → 1 October 2026
- Add 7 days → 8 October 2026
Counting 280 days forward from 1 January 2026 gives the same answer: 8 October 2026.
Adjusting for Longer or Shorter Cycles
Naegele's rule assumes a regular 28-day cycle with ovulation on about day 14. If your cycle is regularly longer or shorter, ovulation shifts too, and so does your due date:
Adjusted due date = LMP + 280 days + (cycle length − 28) days
- With a 32-day cycle, add 4 days: for an LMP of 1 January 2026, the due date becomes 12 October 2026.
- With a 25-day cycle, subtract 3 days: the due date becomes 5 October 2026.
Due Date from Conception Date
If you know when you conceived – for example from ovulation tracking – add 266 days (38 weeks) to the conception date. A conception date of 15 January 2026 gives a due date of 8 October 2026.
IVF Due Date Calculation
IVF pregnancies can be dated very precisely from the embryo transfer:
- Day-5 embryo (blastocyst) transfer: due date = transfer date + 261 days.
- Day-3 embryo transfer: due date = transfer date + 263 days.
For example, a day-5 transfer on 20 January 2026 gives a due date of 8 October 2026.
Dating Scan: When the Due Date Changes
An early ultrasound, ideally between 8 and 13 weeks, measures the baby's crown–rump length (CRL) and is the most accurate way to date a pregnancy. If the scan date differs from your LMP date by more than about 5–7 days in the first trimester, your doctor will usually change your due date to match the scan. Later scans are less accurate for dating because babies grow at different rates.
How to Calculate How Many Weeks Pregnant You Are
Count the days from the first day of your LMP to today and divide by 7. The whole number is the weeks, and the remainder is the days.
Example: LMP on 1 January 2026, today 9 April 2026. That is 98 days, so you are 14 weeks and 0 days pregnant – the first day of the second trimester. Pregnancy weeks are often written as "14+0" or "14w0d". Our days calculator counts the days between any two dates.
Pregnancy Trimesters Explained
| Trimester | Weeks | Key developments and checks |
|---|---|---|
| First trimester | Weeks 1–13 | Organs form, heartbeat seen on ultrasound, dating scan, NT scan at 11–13 weeks, folic acid is essential |
| Second trimester | Weeks 14–27 | Anomaly scan at 18–22 weeks, baby's movements felt, glucose test around 24–28 weeks |
| Third trimester | Weeks 28–40 | Rapid growth, regular check-ups, preparing for delivery |
When Will the Baby Actually Arrive?
Only about 4%–5% of babies are born on their due date. Most arrive within two weeks either side. Doctors use these terms:
- Preterm: before 37 weeks
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks and beyond
Treat your due date as the middle of a "due month" rather than an appointment, and keep your hospital bag ready from about 36 weeks.
Key Pregnancy Milestones by Date
For an LMP of 1 January 2026 (due date 8 October 2026), the main milestones fall on:
| Milestone | Week | Date |
|---|---|---|
| End of first trimester | 13+6 | 8 April 2026 |
| Anomaly scan window starts | 18+0 | 7 May 2026 |
| Third trimester begins | 28+0 | 16 July 2026 |
| Full term begins | 39+0 | 1 October 2026 |
| Estimated due date | 40+0 | 8 October 2026 |
Staying Healthy While You Wait
- Take folic acid (usually from before conception to 12 weeks) and iron and calcium as your doctor advises.
- Register early for antenatal care and keep all check-ups and scans.
- Eat a balanced diet with enough protein, fruit and vegetables; in India the recommended extra intake is about 350 kcal a day in the second and third trimesters.
- Drink enough water – see our guide on how much water to drink.
- Stay active with walking and doctor-approved exercise.
- Seek care immediately for bleeding, severe pain, reduced baby movements, severe headache or swelling.
Early Pregnancy Symptoms and When to Test
Common early signs include a missed period, breast tenderness, tiredness, nausea (often called morning sickness, though it can happen at any time of day), needing to urinate more often and food cravings or aversions. Home pregnancy tests detect the hormone hCG in urine and are most reliable from the first day of a missed period – about 4 weeks after your LMP. If the test is negative but your period still has not arrived, test again after a few days. A blood test at a clinic can detect pregnancy slightly earlier and confirm the result.
Antenatal Check-Up Schedule
In India, antenatal care usually includes at least four visits under the national guidelines, and many doctors see mothers more often:
- First visit: as soon as pregnancy is confirmed, ideally within 12 weeks – blood tests, blood pressure, weight and a dating scan.
- Second trimester: monthly visits, the anomaly scan and a glucose test.
- Third trimester: visits every 2 weeks until 36 weeks, then weekly until delivery.
Government schemes such as the Pradhan Mantri Surakshit Matritva Abhiyan offer free antenatal check-ups on the 9th of every month at government health facilities.
Due Dates for Twins
Twin pregnancies are dated in the same way from the last period or an early scan, but twins usually arrive earlier. Guidelines usually recommend planning delivery between about 36 and 38 weeks, earlier when the twins share one placenta, so the practical "due date" is often two to four weeks before the calculated 40-week date. Your obstetrician will advise on timing based on the type of twin pregnancy.
Frequently Asked Questions
How is the pregnancy due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period. A quick method is LMP + 1 year − 3 months + 7 days, known as Naegele's rule.
How accurate is my due date?
It is an estimate. Only about 4% to 5% of babies are born on their due date, and most arrive within two weeks before or after it. An early dating scan makes the estimate more accurate.
How do I calculate how many weeks pregnant I am?
Count the days from the first day of your last period to today and divide by 7. For example, 98 days is 14 weeks and 0 days.
How do I calculate the due date for IVF?
Add 261 days to the date of a day-5 blastocyst transfer, or 263 days to a day-3 embryo transfer.
What if my periods are irregular?
Naegele's rule is less reliable with irregular cycles. Your doctor will usually date the pregnancy using an early ultrasound instead.
When does the second trimester start?
At 14 weeks 0 days of pregnancy. The first trimester covers weeks 1 to 13, and the third trimester starts at 28 weeks.
How many weeks is a full-term pregnancy?
A full-term pregnancy is 39 weeks 0 days to 40 weeks 6 days. Babies born from 37 weeks are considered early term, and before 37 weeks are preterm.
Can my due date change?
Yes. If an early ultrasound shows the baby is a different size than expected from your last period date, your doctor may update the due date to match the scan.
Is the due date calculated from conception or from the last period?
From the first day of the last menstrual period. Conception usually happens about two weeks later, which is why a pregnancy counted from the LMP lasts 40 weeks but from conception about 38 weeks.
Disclaimer: This article is general information and does not replace medical advice. Your doctor or midwife will confirm your due date and guide your antenatal care.