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Pregnancy Due Date Calculator: Due Date from LMP

What is this tool?

The pregnancy due date calculator estimates your due date, also called the estimated date of delivery, from the first day of your last menstrual period, from a conception date, or from an IVF transfer date. The standard method, known as Naegele's rule, adds 280 days, which is 40 weeks, to the last menstrual period. The other methods use the same 40-week pregnancy length counted from their own starting points, so they should agree when the dates are consistent. Enter your method and date below and you get the due date and its weekday. This page is informational only and is not medical advice; your clinician's date, especially from an early ultrasound, takes precedence over any figure here.

How it works

Naegele's rule is simple: due date equals the first day of the last menstrual period plus 280 days. Conception happens about two weeks after that date in a standard cycle, so the conception method adds 266 days, which is 38 weeks, to the conception date, and it should land on the same due date as Naegele's rule when the conception date is two weeks after the last menstrual period. IVF dates are counted from the transfer date: a day-3 embryo transfer adds 263 days, and a day-5 blastocyst transfer adds 261 days, because the embryo's age at transfer is already counted. The result is always an estimate of 40 weeks, not a promised delivery day.

Worked example

Worked example: last menstrual period 1 March 2026. Adding 280 days gives Sunday 6 December 2026. The same due date comes from a conception date of 15 March 2026 plus 266 days, and from a day-5 IVF transfer on 20 March 2026 plus 261 days. All three methods agree on 6 December 2026, which is a good check that the dates are consistent. A day-3 transfer on 20 March 2026 gives 8 December 2026, two days later, because the embryo was two days younger at transfer.

When to use it

A due date is the anchor for almost all pregnancy planning, which is why it is usually the first date anyone asks for. Maternity leave, childcare arrangements, hospital bookings, announcements and the timing of scans and tests are all planned around it, and each of those plans needs a date to work from, even though everyone involved knows it is an estimate. The due date gives that anchor, and the fact that it may move by days or weeks does not make it useless; it makes it a planning date that can be revised.

It is also the date that lets different methods be compared. A person who knows both a last menstrual period and a conception date, or an IVF transfer date, can check whether the methods agree, and disagreement is itself useful information: it suggests the cycle was not standard, or a date is misremembered, and that is worth raising with a clinician rather than silently picking one figure. The calculator's job is to make that comparison easy, by giving the due date for each method from the same page.

• Planning maternity leave and childcare around an expected date.
• Booking hospital and midwife appointments in the right weeks.
• Checking whether last-period, conception and IVF dates give the same due date.
• Working out how far along a pregnancy is, by counting back from the due date.
• Planning announcements and family arrangements around a realistic window.

The due date should be treated as the middle of a window, not as a single day. Full term runs from 37 weeks to 42 weeks, and babies arrive across that range. A due date that passes without labour is common and is managed by the clinician, not by the calendar, and a due date that is revised after a scan is also common, because the scan measures the pregnancy directly. If your clinician gives you a different date from this calculator, use the clinician's date.

Step by step

Using this page takes one method and one date, and gives a due date that can be checked against the other methods.

1. Choose the method: last menstrual period, conception date, IVF day-3 transfer, or IVF day-5 transfer. Use the method whose date you know most reliably.
2. Enter the date for that method. For the last menstrual period, this is the first day of the period, not the last day.
3. Read the headline result: the estimated due date and its weekday. The weekday is useful for planning, because appointments and leave are usually organised by weekday.
4. Check it against another method if you can. Enter the conception date or transfer date as well and confirm the due dates agree; if they differ by more than a few days, raise it with your clinician.

The weekday matters more than it might seem, because a due date that falls on a weekend is handled differently by some hospitals and employers than one that falls on a weekday, and because leave planning usually starts from a Monday. The calculator shows the weekday for exactly that reason, so the date can be used directly in a plan without a separate calendar check.

One more note: the last menstrual period method assumes a standard 28-day cycle with ovulation around day 14. If your cycle is regularly longer or shorter, the last-menstrual-period date will be a few days out, and the conception method or an ultrasound date will be more reliable. The calculator does not adjust for cycle length, because that adjustment needs information it does not ask for; if your cycle is irregular, treat the last-menstrual-period result as approximate and prefer a scan date.

Common mistakes

The most common mistake is adding nine calendar months instead of 280 days. Nine months from 1 March is 1 December, but 280 days from 1 March is 6 December, because months are not all the same length. The five-day difference is small, but it is exactly the kind of difference that moves a due date across a weekend or past a leave boundary, so the day count matters and the month shortcut should not be used.

The second mistake is using the conception date as if it were the last menstrual period. Adding 280 days to a conception date overstates the pregnancy by about two weeks, because the 280 days are counted from the last menstrual period, not from conception. The conception method adds 266 days, not 280, and mixing the two numbers is the most common source of a due date that looks two weeks too late.

• Adding nine calendar months instead of 280 days, landing a few days early.
• Adding 280 days to a conception date instead of 266 days, landing two weeks late.
• Using the last day of the period instead of the first day as the starting date.
• Using the same rule for day-3 and day-5 IVF transfers, when they differ by two days.
• Treating the due date as a guaranteed delivery day rather than an estimate.
• Ignoring a revised date from an early ultrasound scan.

A further mistake is recalculating the due date every time a new date is learned, and ending up with several different dates in different places. Once a clinician has set a due date, especially from an early scan, that date should be the one used everywhere, even if a fresh calculation from a remembered date gives a different answer. Consistency matters more than any single calculation, because appointments, tests and leave are all booked against one date.

More worked examples and notes

The examples below use the same rules as the worked example, and each can be checked by entering the same method and date into the tool.

• Last menstrual period 1 January 2026: adding 280 days gives 8 October 2026. The same due date comes from a conception date of 15 January 2026 plus 266 days.
• Last menstrual period 1 March 2026: adding 280 days gives Sunday 6 December 2026. This is the worked example from the main part of the page.
• Conception 15 March 2026: adding 266 days gives 6 December 2026, agreeing with the last-menstrual-period method for a standard cycle.
• IVF day-5 transfer 20 March 2026: adding 261 days gives 6 December 2026. IVF day-3 transfer on the same date: adding 263 days gives 8 December 2026.
• Last menstrual period 1 September 2026: adding 280 days gives 8 June 2027. A pregnancy dated from September is due in the following June, which is why due dates cross calendar years so often.
• Full-term window for a due date of 6 December 2026: from 37 weeks, which is 15 November 2026, to 42 weeks, which is 20 December 2026. The due date sits at 40 weeks, in the middle of that window.

The agreement between methods in these examples is the check to aim for. When the last menstrual period, conception and transfer dates all point to the same due date, the dating is consistent and can be trusted as far as any calculation can be. When they disagree, the disagreement is information, not a failure of the calculator, and it is worth taking to a clinician with the dates that produced each figure.

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Frequently asked questions

Written and checked against the calculator above. Last updated: 23 October 2026. Calculation runs entirely in your browser — no date is sent to a server. This tool gives calendar results only, not medical, legal or financial advice. This tool gives informational results only, not medical advice. Confirm your due date with your clinician or midwife.