Pregnancy Due Date Calculator

What the due date calculator returns

It estimates your expected delivery date from the first day of your last menstrual period and your average cycle length, then shows how far along you are today and when each trimester ends. The estimate follows Naegele’s rule.

How the date is calculated

The base rule adds 280 days (40 weeks) to the last menstrual period (LMP). Because the calculator also accepts cycle length, it corrects for cycles that are not the textbook 28 days:

Due date = LMP + 280 days + (cycle length − 28)

A 28-day cycle gets the plain 280 days, a 35-day cycle gets 287, and a 24-day cycle gets 276. Current gestation is derived from the due date by counting backwards 280 days to day zero, then splitting the days elapsed into weeks and days. Trimester milestones are counted from the LMP: the first ends at day 84 (12 weeks), the second at day 196 (28 weeks), and the third at the due date itself.

Worked example: LMP 1 January 2026

With the standard 28-day cycle:

• Due date = 1 Jan 2026 + 280 days = 8 October 2026
• First trimester ends = 1 Jan + 84 = 26 March 2026
• Second trimester ends = 1 Jan + 196 = 16 July 2026

Change the cycle to 35 days and the due date moves a week later, to 15 October 2026, because a later ovulation shifts conception.

If you opened the tool on 1 September 2026 with that example, gestation would read 34w 5d, with 5 weeks to go. That comes from the 37 days remaining to the due date: 280 − 37 = 243 days, which is 34 weeks and 5 days.

Why the due date is a window

Only about 5% of babies arrive on the exact estimated date, and a healthy term pregnancy runs anywhere from 37 to 42 weeks. The EDD is the midpoint of that window, useful for scheduling scans and appointments rather than predicting a birthday. Naegele’s rule assumes a known LMP and a regular cycle; when either is uncertain, an early ultrasound is the more reliable way to date the pregnancy.

Common mistakes

• Using conception date instead of the LMP. The rule is built on the LMP, which falls roughly two weeks before conception.
• Leaving cycle length at 28. If your cycles run long, the default shifts the due date earlier than it should be.
• Treating the EDD as a deadline. It is a midpoint estimate, not a guarantee.
• Ignoring an early scan. A first-trimester ultrasound dates the pregnancy more accurately than any formula.

What if I don’t know the first day of my last period?

Ask your provider for an early ultrasound, which dates the pregnancy from the baby’s size. That measurement becomes the dating standard when the LMP is unknown.

How many weeks is a full pregnancy?

Forty weeks from the LMP, or about 38 weeks from conception. Because a normal pregnancy can run from 37 to 42 weeks, the last few weeks are a range rather than a fixed point.

Does the calculator handle irregular cycles?

Only through the average cycle length you enter. Irregular cycles make the ovulation estimate less reliable, so the due date becomes a rougher guide.

Why do the trimester end dates matter?

Trimester boundaries line up with screening windows and appointment schedules, so they help you plan care rather than predict a birth date.

Can I calculate the due date from conception instead?

Yes, with a different rule. Conception is typically about two weeks after the LMP, so adding 266 days to a known conception date gives a similar estimate. This calculator works from the LMP because that date is easier to pin down; if you only know conception, subtract 14 days to approximate the LMP and use that.

This is an estimate, not medical advice; your provider’s dating is the one that counts. For the fertile window that precedes pregnancy, see the ovulation calculator, and for the weight-gain side, the pregnancy weight gain calculator. The BMI calculator explains the pre-pregnancy BMI bands those guidelines use.