On this page
- How your due date is calculated
- Which due date is most accurate?
- Pregnancy timeline and milestones
- How likely is birth on the due date?
- Due date calendar: quick reference
- Why pregnancy is counted from the last period
- How first-trimester ultrasound dating works
- Term definitions
- Gestational age vs fetal age
- When your cycle is irregular or you are not sure of your dates
- Twins and multiple pregnancies
- Using your due date to plan prenatal care
- When to contact your maternity team
- Frequently asked questions
How your due date is calculated
Your estimated due date (EDD) is the date when you will be 40 weeks pregnant. Pregnancy is counted from the first day of your last menstrual period (LMP) – about two weeks before conception in a typical 28-day cycle – so "40 weeks pregnant" is about 38 weeks after the egg was fertilised.
The due date calendar above can work from four starting points:
| Method | How the due date is calculated |
|---|---|
| Last period | First day of last period + 280 days, adjusted for your cycle length (+1 day for each day your cycle is longer than 28 days, −1 for each day shorter) |
| Conception date | Conception + 266 days (38 weeks) |
| IVF embryo transfer | Day-3 transfer + 263 days; day-5 transfer + 261 days; day-6 transfer + 260 days |
| Ultrasound | Scan date + (280 days − gestational age on the scan) |
It then shows how many weeks and days pregnant you are today, which trimester you are in, and the dates of key milestones. You can copy or share the result.
Naegele's rule
The traditional method, Naegele's rule, adds one year, subtracts three months and adds seven days to the first day of the last period – which is the same as adding 280 days. It assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, ovulation usually happens later, so the calculator moves the due date later by the difference.
Worked examples
- Last period 1 January 2026, 28-day cycle: 1 January + 280 days = 8 October 2026.
- Same period date, 32-day cycle: ovulation is likely about 4 days later, so the due date is 12 October 2026.
- Day-5 embryo transfer on 10 February 2026: 10 February + 261 days = 29 October 2026.
- Ultrasound on 1 April 2026 showing 12 weeks 3 days: 280 − 87 = 193 days later = 11 October 2026.
Which due date is most accurate?
The American College of Obstetricians and Gynecologists (ACOG), with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, recommends in Committee Opinion 700 that the due date be set as soon as the last menstrual period and the first accurate ultrasound are available, and then not changed except in rare circumstances.
Ultrasound in the first trimester – measuring the crown-rump length (CRL) of the embryo or fetus up to 13 weeks 6 days – is the most accurate way to date a pregnancy. ACOG recommends changing an LMP-based due date to the ultrasound date when the difference is larger than these thresholds:
| Gestational age at ultrasound (by LMP) | Change the due date if the difference is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and beyond | 21 days |
The thresholds widen later in pregnancy because babies' sizes vary more as they grow, so a scan becomes less precise for dating. ACOG considers a pregnancy without an ultrasound that confirms or revises the due date before 22 weeks to be suboptimally dated.
For pregnancies conceived by IVF, the due date is set from the embryo's age and transfer date, which are known precisely.
Pregnancy timeline and milestones
| Weeks | What usually happens |
|---|---|
| 0–2 | Counted from the first day of your last period; ovulation and conception happen around the end of week 2 |
| 4 | A home pregnancy test may become positive around the time of the missed period |
| 6–8 | Heartbeat often visible on early ultrasound |
| 8–14 | First-trimester (dating) ultrasound in many countries; screening tests may be offered |
| 12 weeks 6 days | End of the first trimester |
| 18–22 | Detailed anatomy scan |
| 24–28 | Glucose screening for gestational diabetes is commonly done |
| 28 | Third trimester begins |
| 37 | Pregnancy considered "early term" |
| 39 0/7–40 6/7 | "Full term" |
| 40 | Estimated due date |
| 41 0/7–41 6/7 | "Late term" |
| 42 and beyond | "Post-term" |
The calculator shows these milestones as calendar dates for your pregnancy. Exact timings for tests and appointments depend on your country and care provider.
Trimesters
| Trimester | Weeks |
|---|---|
| First | Week 1 to 13 weeks 6 days |
| Second | 14 weeks 0 days to 27 weeks 6 days |
| Third | 28 weeks 0 days to birth |
How likely is birth on the due date?
The due date is an estimate of when you will reach 40 weeks, not a prediction of the day labour will start. Only a small minority of babies are born on their exact due date. Most are born within about two weeks either side of it, and first pregnancies tend to go slightly longer on average. It can help to think of a "due month" – roughly 38 to 42 weeks – rather than a single day.
Due date calendar: quick reference
If your cycle is about 28 days, this table shows the estimated ovulation date and due date for a last period starting on the first of each month in 2026. For any other date, or a different cycle length, use the calculator.
| First day of last period | Likely ovulation (28-day cycle) | Estimated due date |
|---|---|---|
| 1 January 2026 | 15 January 2026 | 8 October 2026 |
| 1 February 2026 | 15 February 2026 | 8 November 2026 |
| 1 March 2026 | 15 March 2026 | 6 December 2026 |
| 1 April 2026 | 15 April 2026 | 6 January 2027 |
| 1 May 2026 | 15 May 2026 | 5 February 2027 |
| 1 June 2026 | 15 June 2026 | 8 March 2027 |
| 1 July 2026 | 15 July 2026 | 7 April 2027 |
| 1 August 2026 | 15 August 2026 | 8 May 2027 |
| 1 September 2026 | 15 September 2026 | 8 June 2027 |
| 1 October 2026 | 15 October 2026 | 8 July 2027 |
| 1 November 2026 | 15 November 2026 | 8 August 2027 |
| 1 December 2026 | 15 December 2026 | 7 September 2027 |
Notice that the due date does not always fall on the 8th: months have different lengths, so "plus 280 days" lands on slightly different days of the month. That is also why the quick "add 9 months and 7 days" version of Naegele's rule can be a day or two out compared with counting 280 days exactly.
Why pregnancy is counted from the last period
It can seem odd that you are considered "two weeks pregnant" before conception has happened. The convention dates from a time when the first day of the last period was the only date most people could be sure of – ovulation and conception happen silently. Counting from the LMP gives everyone the same reference point, and it is still used today so that dates, scan measurements and growth charts all line up. For people who conceive through IVF, the embryo's age is known exactly, so clinicians calculate an equivalent "LMP" date so that the same charts can be used.
How first-trimester ultrasound dating works
In the first trimester, embryos and fetuses grow at a very similar rate, so their size closely reflects their age. The sonographer measures the crown-rump length (CRL) – from the top of the head to the bottom of the trunk – and converts it into a gestational age using standard charts. Early CRL dating is accurate to within roughly five to seven days. After 14 weeks, dating uses a combination of head, abdomen and thigh-bone measurements, which become progressively less precise for dating as natural differences in growth appear.
That is why ACOG recommends setting the due date from the earliest accurate ultrasound and not changing it based on later scans. If a third-trimester scan shows a baby measuring two weeks ahead, it is usually interpreted as a larger-than-average baby, not a wrong due date.
Term definitions
ACOG and the Society for Maternal-Fetal Medicine replaced the single label "term" with more precise categories, because outcomes differ across the 37–42 week window:
| Category | Gestational age |
|---|---|
| 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 0 days and beyond |
| Preterm | Before 37 weeks 0 days |
Your maternity team will discuss what happens if pregnancy continues beyond your due date; practice varies between countries and depends on individual circumstances.
Gestational age vs fetal age
Two different ages are used in pregnancy, which causes confusion:
| Term | Counted from | Typical use |
|---|---|---|
| Gestational age | First day of the last menstrual period | Clinical care, scans, due dates – "you are 20 weeks pregnant" |
| Fetal (conceptional) age | Conception | Embryology, IVF – about 2 weeks less than gestational age |
When this calculator says you are "20 weeks 3 days", it means gestational age, which is how midwives and doctors count.
When your cycle is irregular or you are not sure of your dates
The LMP method works best when you are sure of the date and your cycles are regular. It is less reliable if:
- Your cycles are irregular or longer than about 35 days.
- You became pregnant soon after stopping hormonal contraception or after a recent pregnancy or breastfeeding.
- You had bleeding in early pregnancy that may have been mistaken for a period.
- You do not remember the date.
In these situations, an early ultrasound gives a much better estimate. Use the "Ultrasound" option with the gestational age printed on your scan report.
Twins and multiple pregnancies
The due date is calculated in the same way for twins and triplets, but multiple pregnancies are usually delivered earlier than 40 weeks. Your care team will discuss timing.
Using your due date to plan prenatal care
Once you know your due date, you can map out the typical schedule of prenatal visits. The US Office on Women's Health notes that most experts suggest seeing your doctor or midwife:
| Weeks of pregnancy | Typical visit frequency |
|---|---|
| 4 to 28 | About once a month |
| 28 to 36 | About twice a month |
| 36 to birth | Weekly |
People over 35 or with a higher-risk pregnancy will usually be seen more often. Your own schedule is set by your care team and may differ, but the due date is what every one of these visits is anchored to – which is why getting an accurate date early in pregnancy matters.
Practical planning with your due date
- Work and leave: many employers and benefit schemes ask for the expected due date to calculate leave entitlements. Use the date your care team gives you, which may be adjusted after your dating scan.
- Travel: airlines and insurers often set limits based on how many weeks pregnant you will be on the date of travel. Count forward from your due date (40 weeks) or use the milestone dates the calculator lists to work out how far along you will be.
- The "due month": because only a small share of babies arrive on the due date itself, it can help to think of a window from about 37 to 42 weeks rather than one day – especially when planning visits from family or arranging cover at work.
- Health habits: the same guidance encourages at least 150 minutes (2 hours 30 minutes) of moderate-intensity aerobic activity a week during pregnancy unless your doctor advises otherwise, and avoiding very hot baths and hot tubs.
If your due date changes after an ultrasound, update every plan that depends on it – the new date replaces the old one.
When to contact your maternity team
Contact your midwife, doctor or maternity unit promptly – do not wait for a scheduled appointment – if you have:
- Vaginal bleeding or fluid leaking.
- Severe or persistent abdominal pain.
- Reduced or changed baby movements in the second half of pregnancy.
- Severe headache, vision changes, or sudden swelling of the face, hands or feet.
- Fever, or feeling very unwell.
- Regular contractions before 37 weeks.
Frequently asked questions
How is my due date calculated?
By adding 280 days (40 weeks) to the first day of your last period, adjusted for cycle length, or from conception, IVF transfer or an ultrasound date.
Is the ultrasound due date more accurate than the LMP date?
A first-trimester ultrasound is the most accurate method. ACOG recommends using it to confirm or change the LMP due date when the difference exceeds set thresholds.
Can my due date change?
It is usually set after the first accurate ultrasound and then not changed except in rare circumstances, so later scans that measure ahead or behind normally do not move it.
How many weeks pregnant am I?
Count from the first day of your last period. The calculator shows your current gestational age in weeks and days.
When does the second trimester start?
At 14 weeks 0 days. The third trimester starts at 28 weeks 0 days.
What is the due date for IVF?
Add 261 days to a day-5 blastocyst transfer date, 263 days to a day-3 transfer date, or 260 days to a day-6 transfer date.
How accurate is a due date calculator?
It gives the same result a clinician would get from the same information. The uncertainty comes from the starting information – irregular cycles or an uncertain LMP – which an early ultrasound resolves.
Can I calculate my due date from conception?
Yes. Add 266 days (38 weeks) to the conception date. If you know only the date of intercourse, the conception date is uncertain by a few days because sperm can survive for several days.