Pregnancy Due Date Calculator

Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your due date, how many weeks pregnant you are right now, and which trimester you're in.

How this calculation works

This tool uses Naegele's rule, the standard method doctors and midwives use for a first estimate: due date = LMP + 280 days (40 weeks). It assumes ovulation happens around day 14 of a 28-day cycle, roughly 2 weeks after your last period started.

If your average cycle is longer or shorter than 28 days, ovulation shifts by roughly the same amount, so the calculator adds the difference: due date = LMP + 280 days + (cycle length − 28) days. A 35-day cycle pushes the estimate about a week later; a 21-day cycle pulls it about a week earlier.

Gestational age ("how many weeks pregnant") is simply the number of days since your LMP, converted to weeks and days — not the baby's actual conception age, which is usually about two weeks less.

Worked example

If your last period started on January 1, 2025 and you have a typical 28-day cycle, your estimated due date is October 8, 2025 (280 days later). By April 16, 2025 — 105 days after your LMP — you'd be 15 weeks 0 days pregnant, at the start of the second trimester.

Trimester breakdown

Pregnancy is commonly split into three trimesters based on gestational age (weeks since your LMP):

Trimester	Weeks	What's typically happening
First	0 – 12	Major organs and body systems begin forming; morning sickness and fatigue are common
Second	13 – 26	Often called the "easiest" stage; energy returns and movement ("quickening") is usually felt
Third	27 – 40	Rapid weight gain, engagement of the head, and preparation for labor

Why your due date is an estimate, not a deadline

A due date marks the statistically most likely day, not a guarantee. Full-term birth spans roughly weeks 39 through 40, and "term" more broadly runs from 37 to 42 weeks. Most first-time pregnancies go a few days past the calculated date.

Only about 4–5% of babies are born on their exact due date
About 80% are born within 2 weeks before or after it
First-time mothers tend to deliver slightly later than the estimate on average
An early ultrasound can revise the due date if it differs notably from the LMP-based estimate

LMP dating vs. ultrasound dating

LMP-based dating (what this calculator does) is fast and free but depends on remembering your last period accurately and having regular cycles. Ultrasound dating, especially a crown-rump length measurement between 8 and 13 weeks, is considered the most accurate method and is what most clinicians use to confirm or adjust the due date at your first prenatal visit.

If the two methods disagree by more than about 5–7 days in the first trimester, care providers typically use the ultrasound date going forward.

Frequently asked questions

How accurate is a due date from Naegele's rule?

It's a useful starting estimate, not a deadline. Only about 4–5% of babies are actually born on their calculated due date, and most arrive anywhere from 3 weeks before to 2 weeks after it. An early ultrasound (before 14 weeks) is generally more precise, especially if your cycles are irregular.

What if I don't know the exact date of my last period?

Use your best estimate — even an approximate date gives a useful ballpark. If you're very unsure or have irregular cycles, a dating ultrasound performed early in pregnancy is more reliable than LMP-based calculations.

Why does cycle length change the due date?

Naegele's rule assumes ovulation around day 14 of a 28-day cycle. With longer cycles, ovulation (and conception) happens later, so the due date shifts later too; with shorter cycles it shifts earlier. This calculator adjusts for that automatically.

What's the difference between gestational age and fetal age?

Gestational age counts from the first day of your last period and is what doctors use on charts and scans. Fetal (conceptional) age counts from actual conception, roughly two weeks later. A pregnancy "dated" at 8 weeks gestational age is usually about 6 weeks post-conception.

When does each trimester start and end?

The first trimester runs from week 0 through week 12, the second from week 13 through week 26, and the third from week 27 until birth around week 40. Boundaries vary slightly between sources, but this is the most common convention.

Is my LMP date or cycle length stored anywhere?

No. Everything is calculated locally in your browser — no dates or personal information are sent to a server or saved.

Can this calculator be used for irregular cycles or after fertility treatment?

It's less reliable for irregular cycles, and it doesn't apply to pregnancies from IVF or other assisted conception, where the due date is usually calculated from the transfer or conception date instead. In those cases, follow the date your clinic gives you.

This tool is provided for general information only. Verify important figures independently. · Last reviewed: August 25, 2026