Pregnancy Due Date Calculator (When Will Your Baby Arrive?)
Estimate your due date from your last period or conception date, and see your gestational age, trimester and expected birth window — free and instantly.
Quick samples
Enter a date to see your estimate.
Pick a method, add your date (or try a quick sample), and your due date and today’s gestational age appear here.
Gestational age and the birth window are counted from the first day of your last period (LMP), the same reference your care team uses.
The formula
Due date = LMP + 280 days, adjusted by (cycle length − 28) · or · conception / transfer date + 266 days
- LMP — First day of your last menstrual period — the reference point for dating.
- 280 days — Forty weeks, the standard pregnancy length counted from the LMP.
- 266 days — Thirty-eight weeks from conception, used when the conception or transfer date is known.
- cycle − 28 — Days added or subtracted when your cycle is not the assumed 28 days.
Trimesters and terms at a glance
| Stage | Gestational weeks | Approx. days since LMP |
|---|---|---|
| First trimester | Weeks 0–13 | Day 1–91 |
| Second trimester | Weeks 14–27 | Day 92–195 |
| Third trimester | Weeks 28–40+ | Day 196+ |
| Full-term window | Weeks 39–40 | Day 273–287 |
Gestational age is counted from the first day of your last period, so it runs about two weeks ahead of the embryo's true age.
This free pregnancy due date calculator estimates when your baby is likely to arrive. Enter the first day of your last menstrual period, or your conception or IVF transfer date if you know it, and you get an estimated due date, today's gestational age, your current trimester and the expected birth window.
Your due date is one of the first numbers of pregnancy, and it shapes everything from scan schedules to when you pack your hospital bag. Knowing how it is worked out — and how much to trust it — helps you read the result without stress. It also gives you a shared reference point for every appointment that follows.
The estimate uses the same method midwives and doctors use: Naegele's rule, which adds 280 days, or forty weeks, to the first day of your last period. If your cycles run longer or shorter than the assumed twenty-eight days, the calculator adjusts for that too.
How the due date is calculated
The standard method is called Naegele's rule. You take the first day of your last menstrual period (your LMP), add seven days, subtract three months, and land roughly 280 days later — forty weeks of pregnancy counted from that first day.
Pregnancy is dated from the LMP rather than from conception because most people know the first day of their last period far more reliably than the day they conceived. Since ovulation usually happens about two weeks after the LMP, those first two weeks are included even though you were not yet pregnant.
If you know your conception date — from tracking ovulation, a fertility treatment, or an IVF embryo transfer — the calculator instead adds 266 days, which is thirty-eight weeks from conception. The two routes converge on the same week because they simply start counting from different points.
A worked example
Suppose the first day of your last period was 1 June and your cycles are a regular 28 days. Adding 280 days lands on 8 March, so that is your estimated due date, and you are counted in weeks from 1 June.
If your cycles are consistently 32 days, ovulation tends to happen about four days later, so the true due date is roughly four days further out. The calculator applies that shift automatically: it adds the difference between your cycle and the assumed 28-day cycle to the estimate.
Work the other way and a 24-day cycle pulls the estimate a few days earlier. The date is shown as a specific day, but the honest reading is a window of a few weeks either side, which is how your care team will treat it.
How to use this calculator
Choose whether you are counting from your last period or from a known conception or transfer date. Enter that date, and if you are using the LMP method, set your average cycle length so the estimate matches your body.
Results update instantly, and the quick-sample buttons above load common scenarios so you can see how the output changes before you enter anything real. This is handy if you are still deciding which start date applies to you.
If you are unsure which start date applies, use the first day of the last period you can remember and mention the uncertainty to your midwife. They can confirm or revise the date with a dating scan, which is the most reliable fix for irregular or unknown cycles.
- Use the LMP method if you remember the first day of your last period.
- Switch to conception or IVF transfer if you had a dated fertility procedure.
- Adjust cycle length only when your periods are regularly longer or shorter than 28 days.
Reading your result: gestational age and trimesters
Alongside the due date, the calculator shows your gestational age today — how many weeks and days you are from the first day of your last period. This is the number used at every appointment, on every scan, and in every week-by-week description of pregnancy.
That age also tells you which trimester you are in. The first runs to the end of week thirteen, the second covers weeks fourteen through twenty-seven, and the third begins at week twenty-eight and continues until birth.
You will also see an expected birth window, usually shown from thirty-seven weeks to just over forty-one. Keeping the result as a range matters, because full term spans several weeks and a healthy baby can arrive anywhere inside it.
Knowing your exact week is practical, not just interesting. Screening blood tests, the anatomy scan and glucose checks are each offered in a specific week range, and a reliable gestational age is what slots you into them at the right time.
How accurate is a due date?
Only a small minority of babies — often quoted at around four to five percent — actually arrive on the exact due date. It is best understood as a useful anchor rather than a prediction, and a reminder that most births happen in the weeks around it.
The estimate is also less precise than people assume. Because it relies on a regular twenty-eight-day cycle and typical ovulation timing, the true date can sit several days either side, and a longer or shorter cycle shifts it accordingly.
An early ultrasound, usually done in the first trimester, is the most accurate way to date a pregnancy, and your clinician may revise your due date after it. Where the scan and the calendar disagree, the scan generally wins.
A few signs matter more than the calendar. Regular contractions, your waters breaking, bleeding, or a clear change in your baby's movement are reasons to contact your provider whatever the week — and always sooner if something feels wrong to you.
Key terms
- LMP
- Last menstrual period — the first day of your last period, the reference point for dating a pregnancy.
- Gestational age
- How far along a pregnancy is, counted in weeks and days from the LMP rather than from conception.
- Naegele's rule
- The standard estimate: add 280 days (forty weeks) to the first day of the LMP.
- Term window
- Roughly 37 to 41+6 weeks, the range in which most healthy babies are born.
Pro tips
- If you know your conception or IVF transfer date, use it — it is often more precise than an LMP estimate.
- Enter your true average cycle length; a 32-day cycle shifts the estimate later than a 24-day one.
- Treat the due date as a window, not a deadline — only a small share of babies arrive on the exact day.
Common mistakes
- Counting forty weeks from conception instead of from the last period, which lands about two weeks early.
- Assuming a 28-day cycle when yours is regularly longer or shorter.
- Treating the estimate as certain; only an early ultrasound can truly refine it.
Pregnancy due date calculator — your questions
Why is pregnancy counted from my last period, not from conception?
Because the first day of your last period is usually a date you know, while the exact moment of conception rarely is. Ovulation typically happens about two weeks after that first day, so gestational age runs roughly two weeks ahead of the embryo's true age. It is a convention, and midwives, doctors and scans all use the same reference point so the numbers line up.
What if my cycles are longer or shorter than 28 days?
Then your due date shifts in the same direction. A longer cycle usually means later ovulation and a slightly later due date; a shorter cycle means the opposite. Use the LMP method and set your average cycle length, and the calculator adjusts the estimate by the difference from twenty-eight days. For widely irregular cycles, a dating scan is still the most dependable answer.
How accurate are these due dates?
Accurate enough to be useful, but not exact. Calendar estimates can be off by days to a week or more, and only a few percent of babies arrive on the calculated day. A first-trimester dating scan is the most reliable method, so treat this figure as a well-informed estimate rather than a firm date.
Will my due date change after an ultrasound?
It can. If your scan measurement and your calendar estimate differ by more than the agreed margin, your clinician will revise the due date to match the scan. This is common and not a sign that anything is wrong — it simply reflects more precise information about when the pregnancy began.
What if I don't know my last period or conception date?
Irregular cycles or recent contraception can make the start date genuinely unclear. In that case a dating ultrasound is the best answer, especially in the first trimester. Book an appointment rather than guessing — accurate dating from early on helps track growth and plan any needed care. Your provider may also ask about recent contraception or breastfeeding to narrow the range.
Is the due date different for twins or after IVF?
With IVF, use the embryo-transfer date and the calculator works back to a precise LMP, so the estimate is often more exact than a natural-conception one. For twins or higher-order multiples your team will usually plan an earlier birth, so the single-baby due date becomes a rough guide rather than a target.