Pregnancy Calculator
Due date, how far along you are and every milestone
Only the fields for the method you picked are shown. A first-trimester scan is the most accurate option, so use it if you have had one.
Pregnancy milestones
| Stage | Date | When | What happens |
|---|
How to Use
- 1Pick the date you actually know: period, conception, scan or IVF transfer.
- 2Fill in the one or two fields that appear for it.
- 3Read the due date, then the tiles for how far along you are.
- 4Open the milestone table for scan dates, viability and full term.
How Dating Works
- From last periodLMP + 280 days (Naegele)
- Cycle not 28 daysShift by cycle − 28
- From conceptionConception + 266 days
- From a scanScan date + 280 − age at scan
- From IVFTransfer + 266 − embryo age
Quick Examples
Tap an example to load it above.
Everything about the Pregnancy Calculator
Where the due date comes from, and how much to trust it.
This pregnancy calculator works out your estimated due date from whichever date you actually know: the first day of your last period, the conception date, an ultrasound result or an IVF transfer. It then tells you how far along you are today and when every milestone falls.
All four methods land on the same 280-day timeline, so you can switch between them and compare. Pick the one you have the most reliable date for.
A due date is a midpoint, not a deadline. Fewer than 4 in 100 babies are born on their estimated due date, and only about 60% arrive within a week of it. A normal birth is anywhere from 37 to 42 weeks. Treat the date as the centre of a five-week window.
Why pregnancy is counted from your last period
Gestational age starts on the first day of your last period, roughly two weeks before you actually conceived. It looks odd, but the period date is something you can pin down, while the moment of conception usually cannot be. That is why "6 weeks pregnant" means the embryo is about 4 weeks old, and why every scan, app and midwife appointment uses the same convention this calculator does.
Which method is most accurate?
| Method | Typical accuracy | Best when |
|---|---|---|
| First-trimester ultrasound | ± 5 days | Always, if you have had one |
| IVF transfer date | ± 1 day | The embryo age is known exactly |
| Known conception date | ± 3 days | Ovulation was tracked or timed |
| Last period, regular cycle | ± 1 to 2 weeks | No scan yet |
| Last period, irregular cycle | Unreliable | Wait for a dating scan |
The cycle-length adjustment
Naegele's rule quietly assumes you ovulate on day 14 of a 28-day cycle. Plenty of people do not. If your cycles run to 35 days, ovulation is about a week later, and so is your due date. Set your average cycle length and the calculator shifts the date by the difference. Load the two "Period 1 Jun" examples above to see the week of movement it produces.
What the milestones mean
- 18 to 12 weeks (dating)
The early scan window. A heartbeat is usually visible from around 8 weeks, and the 12-week scan is what clinics use to confirm or revise the due date.
- 220 weeks (anatomy scan)
The detailed mid-pregnancy scan, and the halfway mark of the 40 weeks.
- 324 weeks (viability)
The point from which survival outside the womb becomes possible with intensive care. It is a threshold, not a guarantee.
- 437 to 42 weeks (term)
37 weeks is early term, 39 is full term, 41 is late term and 42 is post-term, the point at which induction is normally discussed.
Nothing you type is sent anywhere
Every date is calculated inside your browser. No period date, scan result or transfer date is transmitted or stored, so the page is safe to use before you have told anyone. This calculator is a calendar tool, not medical advice, so always confirm your dates with a midwife or doctor.
Frequently Asked Questions
How is my due date calculated?
The calculator adds 280 days, or 40 weeks, to the first day of your last period, which is Naegele's rule. If your cycle is not 28 days it shifts the date by the difference, because a longer cycle means you ovulated later. Choosing conception, ultrasound or IVF transfer instead works back to the same 280-day timeline from that known date.
How accurate is an estimated due date?
It is an estimate, not an appointment. Fewer than 4% of babies arrive on their due date and only about 60% are born within a week of it. A dating ultrasound in the first trimester is the most reliable method, which is why clinics often revise the date after the 12-week scan.
What is the difference between gestational age and fetal age?
Gestational age is counted from the dating start, two weeks before conception, and is what every clinic, scan and pregnancy app uses. Fetal age counts from conception itself and is therefore about two weeks less. This calculator reports gestational age, so 12 weeks pregnant means a fetal age of roughly 10 weeks.
How do I use the ultrasound method?
Enter the date of the scan and the gestational age the sonographer reported on that day, in weeks and days. The calculator adds the remaining part of the 280-day timeline to the scan date. This is the most accurate option when the scan was done in the first trimester.
How does an IVF transfer date change the calculation?
An IVF embryo already has a known age, so there is no guesswork about ovulation. A day-5 blastocyst is five days past fertilisation, so the due date is the transfer date plus 261 days; a day-3 embryo gives 263 days and a day-6 blastocyst 260. Pick the embryo age your clinic used.
When does each trimester start and end?
The first trimester runs from the dating start to 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks until birth. The milestone table shows the exact calendar date of each boundary for your pregnancy.
Does a longer or shorter cycle change my due date?
Yes. Naegele's rule assumes ovulation on day 14 of a 28-day cycle. With a 35-day cycle you ovulate about a week later, so the due date moves a week later too. Set your average cycle length and the calculator adjusts automatically.
Is this pregnancy calculator medical advice?
No. It is a free dating tool that does the calendar arithmetic for you, and nothing you enter leaves your browser. Always confirm your due date with a midwife or doctor, since a clinical scan can revise it and only they can advise on your care.