Pregnancy Due Date Calculator

Calculate your estimated due date and see week-by-week milestones and trimester boundaries, based on your last period, an IVF transfer date, an ultrasound scan, or a due date you already know.

How it works

  1. Choose how you want to date the pregnancy — last period, IVF Day-3 or Day-5 transfer, ultrasound scan, or a known due date — and enter the date.
  2. The tool calculates your estimated due date and current gestational week.
  3. Scroll through the week-by-week timeline to see milestones and typical development stages.

Five ways to work out the due date

A pregnancy is dated from the first day of the last menstrual period (LMP), even though conception happens about two weeks later, which is why a full pregnancy is counted as about 40 weeks. The calculator supports five starting points, all of which end up as an estimated due date (EDD):

  • Last menstrual period: EDD = LMP + 280 days (Naegele's rule). Example: an LMP on 1 January 2026 gives 8 October 2026.
  • IVF day 3 transfer: EDD = transfer date + 263 days.
  • IVF day 5 (blastocyst) transfer: EDD = transfer date + 261 days. Example: a day 5 transfer on 10 March 2026 gives 26 November 2026.
  • Known due date: enter the date your clinic or midwife gave you.
  • Ultrasound: EDD = scan date + (280 - gestational age at the scan, in days). Example: a scan on 1 April 2026 that measures 12 weeks and 3 days gives 11 October 2026.

Weeks, trimesters and what the tool shows

Once there is a due date, the tool works back 280 days to find the start of the count and measures from there to today to give your gestational age in weeks and days. The first trimester runs to the end of week 12, the second covers weeks 13 to 26, and the third starts at week 27. The tool marks weeks 20 to 36 as preterm and 37 onwards as term, and shows how many days remain until the due date, or how many have passed. For twins it calculates the due date the same way, but treats 37 weeks as the reference point because twin pregnancies often end earlier than 40 weeks.

The week-by-week cards give approximate size and development milestones, drawn from published guidance from sources such as the NHS and ACOG. The size comparisons are averages and vary from baby to baby. There is also an appointments timeline and a checklist, and your entries are stored on your device only.

How reliable is a due date?

A due date is an estimate, not a deadline. Only a small minority of babies, around one in twenty, arrive on the predicted day, and healthy pregnancies commonly end anywhere from about 37 to 42 weeks. Dates from the last period assume a 28-day cycle with ovulation on day 14, so they are less reliable if your cycles are irregular or you are unsure of the date. Early ultrasound measurement, and IVF transfer dates, which are exact, are generally more accurate for dating. If your midwife or doctor has given you a date, use that one, and enter it as the known due date.

This tool is for information and planning only. It does not replace antenatal care, and it cannot assess the health of a pregnancy. If you have any concerns about your pregnancy, contact your midwife, doctor or local maternity service.

Frequently asked questions

How is the due date calculated from the last period?

The standard method, Naegele's rule, adds 280 days — 40 weeks — to the first day of your last menstrual period, assuming a typical cycle.

Is the calculated date exact?

No — it's an estimate. Only a small percentage of babies arrive on the exact calculated date; your care provider may adjust it based on an ultrasound.

Sources and further reading

Formulas checked against the sources below. Last reviewed . General information only, not professional advice.