Pregnancy due date calculator
Estimate your due date from your last period, conception date, IVF transfer or a dating scan, with gestational age, trimesters and milestones.
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- Nothing you enter is sent
- Free, no account
The first day of bleeding, not the last.
28 days is the textbook assumption. A longer cycle means later ovulation and a later due date.
A due date is an estimate, not an appointment. Only about 4 percent of babies arrive on it. Your midwife or doctor may re-date the pregnancy after a scan, and their date is the one your care will follow.
| Stage | Date | What happens |
|---|---|---|
| 4 weekspassed | 20 Nov 2025 | A missed period — home tests are usually positive from around now. |
| 6 weekspassed | 4 Dec 2025 | A heartbeat is often visible on an early transvaginal scan. |
| 8 weekspassed | 18 Dec 2025 | The embryo is now called a fetus; most first midwife appointments fall between here and 10 weeks. |
| 10 weekspassed | 1 Jan 2026 | Earliest point for non-invasive prenatal testing (NIPT) on maternal blood. |
| 11 weeks | 8 Jan 2026 | Dating and nuchal translucency scan window opens (11+0 to 13+6). |
| 13 weeks 6 days | 28 Jan 2026 | End of the first trimester. |
| 18 weeks | 26 Feb 2026 | Anatomy (mid-pregnancy) scan window opens; runs to about 22 weeks. |
| 20 weeks | 12 Mar 2026 | Halfway. Movements usually become noticeable between 18 and 22 weeks. |
| 24 weeks | 9 Apr 2026 | The threshold of viability in most high-income healthcare systems. |
| 27 weeks 6 days | 6 May 2026 | End of the second trimester. |
| 28 weeks | 7 May 2026 | Third trimester begins. Glucose tolerance testing usually falls just before this. |
| 35 weeks | 25 Jun 2026 | Group B strep swab is commonly offered between 35 and 37 weeks. |
| 37 weeks | 9 Jul 2026 | Early term — no longer classed as premature. |
| 39 weeks | 23 Jul 2026 | Full term begins. Best outcomes cluster in the 39 to 40 week window. |
| 40 weeks — due date | 30 Jul 2026 | The estimated date of delivery. About 4% of babies arrive on it. |
| 41 weeks | 6 Aug 2026 | Late term. Induction is usually discussed around here. |
| 42 weeks | 13 Aug 2026 | Post term. Very few pregnancies are allowed to continue past this point. |
The formula, with your numbers
- Naegele's rule
- due date = first day of last period + 280 days
- Cycle adjustment
- 23 Oct 2025 + 280 days + 0 days (cycle 28 vs 28) = 30 Jul 2026
- Gestational age
- 1 Jan 2026 − 23 Oct 2025 = 70 days = 10w 0d
Gestational age is counted from the first day of the last period, which is about two weeks before conception. That is why you are already called four weeks pregnant when a test first turns positive.
What this means
You are 10w 0d pregnant, in the first trimester, with an estimated due date of Thursday 30 July 2026.
Roughly 80 percent of babies arrive within the two weeks either side of that date, and first pregnancies commonly run a few days past it. The useful window is 37 to 42 weeks — 9 Jul to 13 Aug for you.
If you have had an early scan, use the ultrasound option above: measuring the embryo directly beats a remembered period date, and clinical guidance is to re-date the pregnancy when the two differ by more than about five to seven days.
How to use the due date calculator
- Pick the dating method you have the best information for. If you have had an early scan, use the ultrasound option — it is the most accurate.
- Enter the date. For the last-period method, also set your usual cycle length.
- Read your estimated due date, how many weeks and days pregnant you are today, and which trimester you are in.
- Scroll the milestone table for the dates of scans, tests and term windows.
Every date is calculated in your browser. Nothing you type is transmitted, stored or logged anywhere, and closing the tab discards it.
The four dating methods
| Method | Calculation | Accuracy |
|---|---|---|
| Last menstrual period (Naegele's rule) | LMP + 280 days, adjusted for cycle length | ±1–2 weeks; depends on regular cycles and an accurate recalled date |
| Conception or ovulation date | Conception + 266 days | ±3–5 days if ovulation was tracked properly |
| IVF transfer | Transfer + 280 − 14 − embryo age in days | The most precise of all — embryo age is known exactly |
| Dating ultrasound | Scan date + (280 − gestational age at scan) | ±5 days in the first trimester, widening later in pregnancy |
Naegele's rule and the 28-day assumption
Franz Naegele's rule, published in 1812, adds 280 days to the first day of the last menstrual period. Buried inside it are two assumptions: that your cycle is 28 days, and that you ovulated on day 14. Neither holds for everybody. A 35-day cycle usually means ovulation around day 21, so the true due date is about a week later than the textbook rule gives. This calculator adjusts by the difference between your cycle length and 28 days.
Why IVF dates are exact
In a normal cycle, nobody knows exactly when fertilisation happened. In IVF the clinic does. Gestational age is conventionally counted from two weeks before conception, so the calculation is transfer date + 280 − 14 − the embryo's age. That gives 261 days after a day-5 blastocyst transfer, 263 after a day-3 transfer, and 260 after a day-6 transfer.
When a scan disagrees with your dates
A first-trimester scan measures crown-rump length, which varies very little between healthy embryos at the same age. That makes it more reliable than a remembered period date. Standard obstetric guidance is to re-date the pregnancy when an early ultrasound differs from the LMP estimate by more than about five to seven days. After 22 weeks, scan dating becomes much less precise because growth rates diverge, and the earlier estimate is usually kept.
Understanding gestational age
Gestational age is counted from the first day of your last period — roughly two weeks before conception actually occurred. It is a historical convention from when the period date was the only knowable landmark, and it is why you are already described as "four weeks pregnant" when a test first turns positive, despite the embryo being about two weeks old.
Ages are written as weeks + days, so "12+4" means twelve weeks and four days. Fetal age (post-conception age) is always two weeks less than gestational age.
Trimesters
| Trimester | Weeks | What is going on |
|---|---|---|
| First | 0+0 to 13+6 | All major organ systems form. Nausea and fatigue peak around weeks 8–10 and usually ease by 14. |
| Second | 14+0 to 27+6 | The anatomy scan, first felt movements, and for most people the most comfortable stretch. |
| Third | 28+0 to birth | Rapid fetal growth and weight gain, more frequent monitoring, and preparation for birth. |
Term definitions
The American College of Obstetricians and Gynecologists replaced the single "term" label with four categories in 2013, because outcomes differ measurably across them:
| Label | Gestational age |
|---|---|
| Preterm | Before 37+0 |
| Early term | 37+0 to 38+6 |
| Full term | 39+0 to 40+6 |
| Late term | 41+0 to 41+6 |
| Post term | 42+0 and beyond |
How likely is the due date?
Only about 4 percent of babies are born on their estimated date, and roughly 80 percent arrive within the two weeks either side of it. For a first pregnancy the most common arrival point is several days after 40 weeks; subsequent pregnancies tend to run slightly shorter. The due date is best understood as the middle of a distribution, not a deadline.
What this calculator cannot do
It works from dates alone. It cannot account for irregular cycles you have not measured, multiple pregnancies (twins are usually delivered earlier), medical conditions affecting gestation length, or a planned induction or caesarean. Your midwife or doctor will set the date your care actually follows, and if they have re-dated you after a scan, use their number.
Questions
How is a due date actually worked out?
Naegele’s rule adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle with ovulation on day 14. This calculator adjusts that assumption if your cycle is longer or shorter.
Why does my scan give a different date?
A first-trimester scan measures the embryo directly and is more accurate than a remembered period date. Clinical guidance is to re-date the pregnancy when an early scan differs from the LMP estimate by more than about five to seven days.
How do IVF due dates work?
Because the embryo’s age is known exactly, the due date is the transfer date plus 280 days minus 14 days minus the embryo’s age — 261 days after a day-5 blastocyst transfer, 263 after a day-3 transfer.
How likely is the baby to arrive on the due date?
About 4 percent of babies are born on their estimated date. Roughly 80 percent arrive within the two weeks around it, and a first-time pregnancy commonly runs a few days past 40 weeks.
Is anything I type here sent anywhere?
No. The dates stay in your browser tab — nothing is uploaded, stored or logged, and closing the tab discards them.