Due Date Calculator
Find out exactly when your baby is due. Enter your last period date, conception date or IVF transfer date — we'll map your due date, every key milestone, and when to book each scan.
Calculate your due date
Pick the date you know. The calculator does the rest.
Cycle length adjustment (for irregular or long/short cycles)
Your pregnancy journey
Your key milestone dates
Now plan your scans
Use the Scan Timing Calculator to see which pregnancy scans are available for your stage — with exact booking windows.
How your due date is calculated
A typical pregnancy lasts 280 days — exactly 40 weeks — measured from the first day of your last menstrual period (LMP). Adding 280 days to that date gives your estimated due date (EDD). This is Naegele's rule, the same formula your midwife, sonographer and obstetrician will use.
Pregnancy is dated from LMP rather than from conception because very few women know the exact day they conceived — but most can recall when their last period started. It also means gestational age is always about two weeks longer than the embryo's actual age. A "six-week" pregnancy contains a four-week-old embryo.
If you know your conception date or had IVF, the calculator uses those inputs directly. IVF gives the most accurate due date of all because fertilisation happens in the laboratory on a known day. For irregular cycles, the cycle-length adjustment shifts your notional LMP to reflect later or earlier ovulation — a woman on a 35-day cycle typically ovulates around day 21, not day 14.
Only 5% arrive on time
Just 5% of babies are born on their estimated due date. About 80% arrive within two weeks either side — between 38 and 42 weeks. An EDD is a reference point, not a deadline.
A dating scan between 7 and 13 weeks gives a more accurate EDD than any date-based calculation. If the scan disagrees with the calculator by more than five days, the scan wins.
How accurate is this due date?
Accuracy depends on what you put in. The gold standard is still an ultrasound measurement, but some inputs come very close.
Key pregnancy milestones
The gestational ages below are fixed clinical markers used by all NHS and private providers in the UK.
| Milestone | Gestation | Clinical significance |
|---|---|---|
| End of first trimester | 13 weeks | Risk of miscarriage falls significantly |
| Quickening (first flutters) | 16–22 weeks | First perceived fetal movements |
| Anomaly scan window | 18–22 weeks | Detailed structural check of every major organ |
| Viability | 24 weeks | Baby has a realistic chance of survival if born |
| End of second trimester | 27 weeks | Third trimester begins |
| Full term | 37 weeks | Baby considered fully matured; birth no longer preterm |
| Estimated due date | 40 weeks | Midpoint of the normal birth window |
| Post-term | 42 weeks | Induction is usually recommended by this point |
When your due date may change
A dating scan is the most accurate way to confirm an EDD. If it disagrees with the calculator, the scan takes precedence.
Frequently asked questions
How is the due date calculated?
The calculator uses Naegele's rule: 280 days (40 weeks) added to the first day of your last menstrual period. For IVF pregnancies, it works backward from the embryo transfer date, accounting for whether it was a day-3 or day-5 transfer. For known conception dates, it adds 266 days (38 weeks) since conception happens roughly two weeks after LMP.
What percentage of babies are born on their due date?
Only about 5%. Around 80% of babies arrive within two weeks either side — between 37 and 42 weeks — and only around 4% reach 42 weeks without intervention. The EDD is the midpoint of the normal delivery window, not a deadline.
My periods are irregular. Can I still use this calculator?
Use the cycle-length adjustment in the advanced options to improve the estimate. But for irregular cycles the result is unreliable — ovulation doesn't consistently happen on day 14. The only accurate way to establish gestational age with irregular cycles is a dating scan between 7 and 13 weeks.
The calculator and my dating scan disagree. Which is right?
The scan. A dating scan measures your baby's crown-rump length directly and is accurate to within three to five days between weeks 7 and 13. If the two disagree by more than five days, clinical practice is to re-date the pregnancy from the scan measurement — and that becomes your official EDD.
How accurate is the IVF due date?
Very accurate. Because conception happens in the laboratory on a known day, an IVF-derived due date is typically within one day of the true gestational age. Day-5 blastocyst transfers and day-3 transfers use slightly different offsets, both of which the calculator handles.
Does this work for twin pregnancies?
The due date calculation is identical for singletons and twins. However, most twin pregnancies are delivered earlier — usually between 36 and 37 weeks for dichorionic twins and earlier for monochorionic twins. The 40-week EDD still serves as the reference point, but the actual delivery window is typically three to four weeks earlier.
Book a dating scan at IUS London
A dating scan between 7 and 13 weeks measures your baby directly — the clinical gold standard for pregnancy dating. Same-week appointments available in Kensington.