Pregnancy Calculator
Get the number you need, with the working shown.
What is the Pregnancy Calculator?
The Pregnancy Calculator estimates your due date and current gestational age using Naegele's rule — last menstrual period plus 280 days — with two alternative modes for a known conception date or an IVF embryo transfer date. Enter one date and it returns your current week and day of pregnancy, which trimester you're in, and a list of key milestones (end of first trimester, viability, full term, due date) mapped onto real calendar dates. Every calculation shows its method directly, and the result comes with an honest reminder that a due date is a statistical estimate, not a prediction anyone should expect to be exact.
Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14 — a real but imperfect assumption, since cycle length varies significantly between people and even cycle to cycle for the same person. The conception-date and IVF-transfer modes exist specifically because they sidestep that assumption: if you know the actual date of conception or embryo transfer, the calculation is more directly tied to your specific pregnancy rather than an average cycle.
Why use a free pregnancy calculator?
Three calculation modes for different situations
Last menstrual period (LMP), a known conception date, or an IVF transfer date (day-3 or day-5) — each uses the offset appropriate to what you actually know.
Gestational age shown in weeks and days
Get your exact current point in the pregnancy — "24 weeks, 3 days" — the same format used on clinical charts and by care providers.
Key milestones mapped to real dates
End of first trimester, a commonly cited viability marker, start of third trimester, full term and due date are all converted to actual calendar dates, not just week numbers.
IVF-specific offsets
Day-3 and day-5 (blastocyst) transfers use their own specific day offsets rather than being forced through the LMP formula, since the transfer date maps to a known embryonic age.
Honest about uncertainty
The result states plainly that a due date is a statistical estimate and most births don't land on it exactly — not a promise dressed up as a fact.
How do you use the Pregnancy Calculator?
- 1
Choose your calculation mode
Pick "Last period" if you know the first day of your last menstrual period, "Conception date" if you know or estimate when conception occurred, or "IVF transfer date" if this pregnancy started via IVF.
- 2
For IVF, select the transfer type
Day-3 transfer and day-5 (blastocyst) transfer use different offsets to the due date, since a day-5 embryo is two days further along than a day-3 embryo on the same calendar date.
- 3
Enter the relevant date
The date must be in the past — today or earlier — since none of the three modes can calculate from a future date.
- 4
Read your gestational age and due date
Your current week and day, trimester, and estimated due date appear immediately, along with the full list of milestone dates below.
How is pregnancy calculator calculated?
Due date = LMP + 280 days, or Conception date + 266 days, or IVF transfer date + 261–263 daysWhere:
- LMP
- First day of the last menstrual period — the standard clinical starting point for dating a pregnancy
- 280 days
- Naegele's rule: 40 weeks from LMP, assuming a 28-day cycle with ovulation on day 14
- Conception date + 266 days
- 38 weeks from conception — 14 days less than the LMP method, since conception is assumed to occur roughly two weeks after LMP
- IVF transfer + 261 or 263 days
- A day-5 (blastocyst) transfer embryo is 5 days old at transfer, so due date = transfer + 261 days; a day-3 embryo is 3 days old, so due date = transfer + 263 days
The simple version
Naegele's rule is the classic method: take the first day of your last period and add 280 days (40 weeks) to estimate the due date. If you know your actual conception date instead, the calculator adds 266 days (38 weeks), since conception typically happens about two weeks after LMP in a standard cycle. For IVF, the embryo's exact age at transfer is known precisely, so the calculator works backward from that known age instead of guessing a cycle date at all.
What does it look like in practice?
Last menstrual period mode
Input
First day of last period: 1 May 2026
Result
Due date: 5 February 2027
1 May 2026 + 280 days = 5 February 2027, following Naegele's rule exactly.
Known conception date
Input
Conception date: 15 May 2026
Result
Due date: 5 February 2027
15 May 2026 + 266 days = 5 February 2027 — the same result as the LMP example above, because 15 May is exactly 14 days after 1 May. The two formulas are built to agree exactly when conception falls on day 14 of the cycle, which is the assumption Naegele's rule makes implicitly.
IVF day-5 blastocyst transfer
Input
Transfer date: 10 May 2026, day-5 transfer
Result
Due date: 26 January 2027
10 May 2026 + 261 days = 26 January 2027. Gestational age is measured from an LMP-equivalent anchor date, calculated by working backward 280 days from this due date, so it lines up with standard week-based pregnancy tracking.
How accurate is it?
Only about 5% of babies are born on their exact calculated due date — most arrive within roughly two weeks either side of it. Naegele's rule is a widely used clinical starting estimate, not a prediction with any claim to precision for an individual pregnancy, and this calculator states that plainly rather than presenting a single date as more certain than it is.
- Naegele's rule assumes a standard 28-day menstrual cycle with ovulation on day 14. Cycles shorter or longer than 28 days shift the true conception date relative to LMP, which is why a clinical ultrasound dating scan, usually done in the first trimester, is considered more accurate than an LMP-based estimate alone.
- The conception-date mode removes the cycle-length assumption but depends on knowing the conception date accurately, which is often an estimate itself unless conception was medically tracked.
- IVF transfer dates are the most precise input this calculator accepts, since the embryo's exact age is known at the moment of transfer — but the due date is still a statistical estimate of a typical pregnancy length, not a guarantee.
- The "viability milestone" shown among the key dates is a commonly cited medical and legal reference point, not a guarantee for any individual pregnancy — outcomes at that gestational age vary considerably case by case.
What should you know before using it?
The three trimesters and key milestones, by week of pregnancy:
| Milestone | Approximate week |
|---|---|
| First trimester | Weeks 1–13 |
| End of first trimester | Week 13 |
| Second trimester | Weeks 14–27 |
| Viability milestone | Around week 24 — a commonly cited marker, not a guarantee |
| Third trimester | Weeks 28–40 |
| Full term | Week 37 |
| Due date | Week 40 |
Which tools relate to the pregnancy calculator?
The pregnancy calculator is one of 21 calculators on this site. These are the ones most often used alongside it — either because they handle the next step in the same job, or because they answer a question this tool raises.
Age Calculator
Uses the same real-calendar date arithmetic this calculator relies on, applied in the opposite direction — from a birth date forward, instead of from LMP forward to a due date.
BMI Calculator
Standard BMI categories are not designed for use during pregnancy — a separate consideration worth knowing before checking your weight against the usual adult chart.
Calorie Calculator
Calorie needs increase during pregnancy beyond what a standard TDEE calculation accounts for — check with a healthcare provider for pregnancy-specific guidance rather than relying on the general formula.
When should you use a pregnancy calculator?
Because the pregnancy calculator runs entirely in your browser, it suits work you would not want to hand to a third-party server — client files, unpublished drafts, anything under an NDA. These are the situations people reach for it in most often.
Estimating a due date early in pregnancy
Get an initial estimate from your last period or a known conception date before a clinical dating ultrasound provides a more precise figure.
Tracking gestational age week by week
Check exactly how many weeks and days along you are at any point, useful for comparing against week-by-week pregnancy information and prenatal appointment schedules.
IVF pregnancy dating
Calculate a due date precisely from a known embryo transfer date, without relying on an LMP that may not reflect a medicated or non-standard cycle.
Planning around key milestones
See real calendar dates for the start of the third trimester or the full-term mark, useful for planning leave, travel restrictions or hospital pre-registration.
Frequently asked questions
How accurate is a pregnancy due date calculator?
What is Naegele's rule?
Why does the conception-date method give a different due date than the LMP method?
How is a due date calculated for an IVF pregnancy?
What week does the third trimester start?
What does the "viability milestone" mean?
Is this pregnancy calculator medical advice?
Is the pregnancy calculator safe to use?
This tool runs entirely inside your browser. Your text, files and settings are processed on your own device and are never uploaded to our servers — there is nothing for us to store, log or leak. You can confirm it yourself: open your browser DevTools, switch to the Network tab, and use the tool. You will not see an upload request. It also means the tool keeps working if your connection drops mid-task.
Where do these figures come from?
Every method, threshold and standard this page relies on, with a link to the document that defines it. Check them — a tool that will not show its sources is asking you to take its word for it.
- 1Methods for Estimating the Due Date
American College of Obstetricians and Gynecologists (ACOG), Committee Opinion No. 700 · 2017
Supports: Naegele's rule (LMP + 280 days), the conception-date and IVF-transfer dating methods, and the statement that only about 5% of babies are born on their exact calculated due date.
- 2WHO recommendations on antenatal care for a positive pregnancy experience
World Health Organization · 2016
Supports: The statement that an early ultrasound dating scan is generally more accurate for gestational age than an LMP-based estimate alone.
Read more about pregnancy calculator
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