Due Date Calculator

What is a due date?

A due date calculator estimates the day your baby is expected to arrive, counting from the first day of your last menstrual period. It is the single most-quoted date of a pregnancy — written on every medical record, used to schedule scans and tests, and counted down to by the whole family. Yet it surprises many parents to learn it is an estimate with a wide margin, not a deadline.

How it works

The calculator uses Naegele's rule, the standard obstetric formula used worldwide for over 200 years:

Due date = first day of last period + 280 days (40 weeks)

The 280 days assume a regular 28-day cycle. It also shows days remaining — the countdown from today to the due date — which updates every day you visit.

Quick mental version

Naegele's rule is often taught as: add 7 days, subtract 3 months, add 1 year to the LMP. For an LMP of 1 January: 1 + 7 = 8 January → minus 3 months = 8 October → plus 1 year = 8 October — the same answer as adding 280 days.

How to use this calculator

1. Enter the first day of your last menstrual period.
2. Read your estimated due date and how many days remain.
3. Use the date to plan: the week-20 scan lands around the midpoint, maternity arrangements in the final month.

Worked example

Last period began 1 January:

• Due date = 1 Jan + 280 days = 8 October
• If today is 1 July, days remaining ≈ 99 days

Limitations and tips

• Only ~5% arrive on the day: the great majority of babies arrive within two weeks before or after the due date. Full term is 37–42 weeks — all of it normal.
• Irregular cycles: Naegele's rule assumes a 28-day cycle. Longer or irregular cycles shift the true date; a first-trimester dating ultrasound is the most accurate method.
• It can be revised: doctors often adjust the due date after the dating scan — the scan date then becomes the official one. That is normal, not a problem.
• Plan a window, not a day: have bags packed and arrangements ready from week 37. Babies rarely consult the calendar.

Milestones to look forward to

Counting down is easier with markers: around week 12 the first-trimester screening, week 20 the detailed anomaly scan where many parents learn the sex, week 28 the start of the third trimester, and week 37 — officially "early term," when it is wise to have everything ready. Each milestone passed is real progress worth celebrating.