How the due date calculator works
This calculator uses the same formulas obstetricians use:
- From your last menstrual period (LMP): due date = LMP + 280 days. This is Naegele's rule — the standard formula based on a 28-day cycle with ovulation on day 14.
- From conception date: due date = conception date + 266 days.
- From IVF transfer: due date = transfer date + 266 days − age of embryo at transfer (so a day-5 transfer adds 261 days).
If your cycle is regularly longer or shorter than 28 days, the LMP calculation adjusts automatically: it adds (cycle length − 28) days to the standard 280-day result.
How accurate is a calculated due date?
Only about 5% of babies are born on their exact estimated due date. The vast majority arrive within a two-week window before or after. A pregnancy is considered full term from 37 weeks, early term at 37+0 to 38+6, full term at 39+0 to 40+6, late term at 41+0 to 41+6, and post term from 42 weeks.
Your dating ultrasound at 10–13 weeks is more accurate than calculator-based dates, especially if you have irregular cycles, late ovulation, or are unsure of your LMP. If the ultrasound differs from the LMP-based date by more than about 5–7 days, midwives usually go with the ultrasound estimate.
Pregnancy timeline at a glance
| Trimester | Weeks | What's happening |
|---|---|---|
| First | 1–13 | Conception, implantation, organ formation, neural tube closes. First trimester guide |
| Second | 14–27 | Anatomy scan, first kicks, rapid growth, "golden period." Second trimester guide |
| Third | 28–40 | Fetal weight gain, kick counts, hospital bag, signs of labour. Third trimester guide |
Key milestones — when to expect them
Weeks 6–8
First antenatal appointment ("booking-in"). Pregnancy confirmation, medical history, blood tests, urine sample, midwife allocation.
Weeks 10–13
Dating ultrasound — confirms gestational age, may revise your due date. Combined screening for chromosomal conditions if you choose. End of T1.
Week 16
Some women feel first flutters ("quickening"). Routine antenatal check.
Weeks 18–22
Anatomy scan — detailed ultrasound checking baby's organs and growth. You can usually find out the sex if you choose.
Weeks 24–28
Glucose tolerance test for gestational diabetes. Anti-D injection if Rh-negative. Whooping cough vaccine. Start of T3.
Week 28
Start daily kick counts. Antenatal classes typically start.
Weeks 35–37
Pack your hospital bag. Discuss your birth plan at the 36-week appointment. Position scan if needed.
Weeks 37–42
Watch for signs of labour. Most babies arrive between 38 and 42 weeks. Induction is usually offered around 41–42 weeks if labour hasn't started.
Why due dates change
It's common — and not concerning — for a due date to be revised once or twice during pregnancy. The reasons:
- The dating ultrasound is more accurate than LMP-based dating. If it differs by more than 5–7 days, the ultrasound date is used.
- You ovulated later than day 14 — common with irregular cycles or after stopping hormonal contraception. The dating scan corrects for this.
- You weren't sure of your LMP date when calculating.
- Recent miscarriage or pregnancy loss can disrupt your cycle and make LMP-based estimates unreliable.
After the dating scan, your due date is fixed for the rest of pregnancy — even if later ultrasounds suggest the baby is measuring big or small for dates. This is because growth varies but the dating ultrasound is the most accurate measure of when the pregnancy started.
Trying to conceive? Use a different calculator
If you're not pregnant yet but trying, an ovulation calculator (typically based on cycle length and last period) is more useful. Most cycles ovulate roughly 14 days before the next period — so a 32-day cycle ovulates around day 18, not day 14. Tracking basal body temperature, ovulation predictor kits or cervical mucus all give a more accurate picture than calculator estimates alone.
What this calculator can't tell you
- The exact day your baby will be born — only ~5% of babies arrive on their estimated due date
- The sex of the baby — that's the anatomy scan at 18–22 weeks
- Whether a pregnancy is healthy — that's your antenatal care
- Whether you're carrying twins or triplets — that's the dating ultrasound
The calculator is a starting point. Your midwife and ultrasound technicians will give you everything else.
Inside Baby Novum: save your due date in the app to unlock daily fetal development updates, week-by-week articles, kick counts, contraction timer and a built-in birth plan builder.
Related guides
FAQ — Pregnancy due date
How is a pregnancy due date calculated?
The most common method is Naegele's rule: due date = first day of your last menstrual period (LMP) + 280 days (40 weeks). This assumes a regular 28-day cycle with ovulation on day 14. If you know your conception date, due date = conception date + 266 days (38 weeks).
How accurate is a due date calculator?
Calculator-based due dates are estimates. Only about 5% of babies are born on their due date — most arrive within two weeks before or after. Your dating ultrasound (typically at 10–13 weeks) is the most accurate measurement and may revise your due date.
What if my cycle is not 28 days?
If your cycle is regular but longer or shorter than 28 days, you can adjust: add (your cycle length minus 28) days to the standard due date. For example, a regular 32-day cycle adds 4 days. The LMP form on this page does this automatically when you change the cycle-length field.
Can the calculator work without my LMP?
Yes — if you know your conception date (e.g. from ovulation tracking) or your IVF transfer date, use the relevant tab on the calculator. For IVF, choose your transfer date and embryo age (day-3 or day-5).
Why does my dating ultrasound give a different due date?
An early ultrasound (10–13 weeks) measures the baby's crown-rump length, which gives a more accurate gestational age than the LMP — especially if you have irregular cycles, late ovulation, or aren't sure of your last period. If the ultrasound estimate differs by more than 5–7 days, the ultrasound date is usually used.
Is a pregnancy 9 months or 40 weeks?
A standard pregnancy is 40 weeks from the first day of your last period — which is closer to 9.2 calendar months. The "9 months" figure is a rough approximation. Counting from conception, pregnancy is 38 weeks (about 8.7 months).
Can my due date be wrong?
Yes — it's an estimate. The most accurate dating tool is the early ultrasound. After that, your due date is fixed for the rest of pregnancy. Late-pregnancy ultrasounds measure growth, not gestational age.
The pregnancy due date calculator — at a glance
A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — Naegele's rule. If you know your conception date instead, add 266 days. For IVF transfers, the calculation is based on transfer date and embryo age. Calculator-based dates are estimates: only about 5% of babies arrive on the exact date, and your dating ultrasound at 10–13 weeks may revise the estimate. After the dating scan, your due date is fixed for the rest of pregnancy. Use this calculator as a starting point, then build the rest of your pregnancy timeline around your antenatal appointments — booking in at 8–10 weeks, dating scan at 10–13, anatomy scan at 18–22, glucose tolerance test at 24–28, and the run-up to birth from week 36 onward. Most babies arrive between 38 and 42 weeks. Knowing your due date is the gateway to every other pregnancy milestone — week-by-week growth, the right time to start kick counts, when to pack your hospital bag, and when to write your birth plan.