Is a Pregnancy Calculator Accurate?
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: Is a pregnancy calculator accurate? At the arithmetic, yes, but it's an estimate. It adds 280 days to your last period and assumes ovulation on day 14 of a 28-day cycle. If that fits you, it's usually close to a scan date. With irregular or long cycles it can be a week or more out.
Here's what calculators assume, how cycle length and other factors skew the result, how accurate ultrasound dating is by trimester, and which date to trust. To get your estimate now, try the due date calculator.
What pregnancy calculators assume
Almost every calculator, including the NHS one, starts from the first day of your last menstrual period (LMP). Behind that are four assumptions:
| Assumption | Reality |
|---|---|
| Your cycle is 28 days | Many people have shorter, longer or variable cycles |
| You ovulated on day 14 | Ovulation can move around, even with regular cycles |
| You remember your LMP correctly | People often estimate, especially if periods are light or irregular |
| Pregnancy lasts 280 days | That's an average; most healthy pregnancies end between 37 and 42 weeks |
The formula itself, LMP + 280 days, is simple and correct. The weak point is whether those assumptions match your body. Our guide to how is due date calculated explains the formula and Naegele's rule.
How irregular or long cycles change the result
If your cycle is longer than 28 days, you probably ovulated later than day 14. Your baby is then a bit younger than the calculator thinks, and an early scan may move your due date later. The opposite happens with short cycles.
| Usual cycle length | Likely ovulation | Effect on LMP-based due date |
|---|---|---|
| 24 days | Around day 10 | About 4 days too late; scan may move it earlier |
| 28 days | Around day 14 | No adjustment |
| 32 days | Around day 18 | About 4 days too early; scan may move it later |
| 35 days | Around day 21 | About 7 days too early; scan may move it later |
| Irregular | Unpredictable | LMP date may be well off; rely on the scan |
Some calculators, including ours, let you enter your cycle length and shift the date. That helps, but it's still an estimate, because ovulation isn't fixed even in a regular cycle.
Other situations where an LMP calculator is less reliable:
- you're not sure when your last period started
- you had bleeding in early pregnancy that may not have been a period
- you got pregnant soon after stopping hormonal contraception
- you got pregnant while breastfeeding, before your cycles had settled
How accurate is ultrasound dating?
In the first trimester, babies grow at a very similar rate, so measuring crown to rump length is a reliable guide to age. ACOG calls an ultrasound up to 13 weeks 6 days the most accurate way to establish or confirm gestational age. The NHS offers a dating scan at about 10 to 14 weeks for this reason.
Later in pregnancy, healthy babies vary a lot in size, so a scan becomes less precise for dating:
| When the scan is done | Typical dating precision | ACOG: move the due date if the scan differs from LMP by more than |
|---|---|---|
| Up to 8 weeks 6 days | Highest | 5 days |
| 9 weeks to 13 weeks 6 days | High | 7 days |
| 14 to 15 weeks 6 days | Good | 7 days |
| 16 to 21 weeks 6 days | Moderate | 10 days |
| 22 to 27 weeks 6 days | Lower | 14 days |
| 28 weeks and later | Lowest | 21 days |
Guidance from one NHS trust (Royal Berkshire) puts the scan's margin of error at about 5 days either way until 16 weeks and about 10 days after that. ACOG adds that a pregnancy without a scan confirming the date before 22 weeks is "suboptimally dated".
When to trust which date
- IVF pregnancy: trust the date from your embryo transfer. It's based on a known embryo age, and clinics don't usually change it after scans. See the IVF due date calculator.
- First-trimester scan done: trust the date in your notes. If it differs from your LMP by more than the margins above, the scan date is the one to use.
- No scan yet: use your LMP date as a working estimate, ideally adjusted for cycle length, and book a scan.
- Only a late scan: your provider may use it, but it's less precise. Ask how they've dated your pregnancy.
Once the due date is set from an early scan, ACOG says it shouldn't normally change again. Later scans check growth, not dates. Read can your due date change? for more.
Even the best date is a window
No method can tell you the day your baby will arrive. NHS guidance says only about 1 in 20 babies are born on their due date, and most arrive between 37 and 42 weeks. A more accurate due date mainly helps your care team time screening tests, judge growth, and decide when to offer induction if you go overdue.
How to get the most accurate estimate
- Write down the first day of every period when you're trying to conceive.
- Know your usual cycle length and enter it in the calculator.
- If you tracked ovulation, try the due date calculator from conception as a cross-check.
- Book your first appointment early; see the pregnancy appointment schedule.
- After your dating scan, update every app and calendar to the new date.
- Check your current week with How Many Weeks Pregnant Am I?
Use your real date in Momi
Momi estimates your due date from your last period or a conception date, then follows your pregnancy week by week. When your dating scan confirms or changes the date, update it so your baby's size, your week and your countdown stay accurate. You can also save your scan photos and log appointments.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: are pregnancy apps accurate? · why pregnancy calculators add 2 weeks
Sources
- ACOG, Committee Opinion 700: Methods for Estimating the Due Date
- NHS, Pregnancy due date calculator
- NHS, 12-week scan
- Royal Berkshire NHS Foundation Trust, Baby due dates and how it affects my care (PDF)
- ACOG, Ultrasound Exams
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
Is a pregnancy calculator accurate?
It's accurate at the maths, but only as good as its assumptions. A calculator adds 280 days to your last period and assumes you ovulated on day 14. If your cycle is regular and close to 28 days, it's usually within about a week of a scan date. A first-trimester scan is more accurate.
Can pregnancy calculators be wrong?
Yes, if the inputs or assumptions don't fit you. Irregular or long cycles, an uncertain period date, bleeding in early pregnancy or recently stopping the pill can all push the estimate out by a week or more. That's why providers confirm the date with an ultrasound.
How accurate is a dating scan?
Very. ACOG says a first-trimester ultrasound up to 13 weeks 6 days is the most accurate way to set gestational age. One NHS trust's guidance gives a margin of about 5 days either way until 16 weeks, and about 10 days after that.
Are online due date calculators all the same?
Most use the same 280-day rule, but they differ in details: whether they adjust for cycle length, whether day one counts as day 0 or 1, and whether they accept conception or IVF dates. That's why two calculators can be a day or more apart.
Which due date should I use?
Use the date in your maternity notes. That's usually from your dating scan, or your IVF transfer date if you had IVF. Update your app to match so your weeks, baby size and reminders line up with your care.
Related guides
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
