How is the due date estimated?

With Naegele’s rule: the calculator adds exactly 280 days — 40 weeks — to the first day of your last menstrual period. This is the same convention doctors and midwives use for initial dating before an ultrasound refines it.

What if my cycles are longer or shorter than 28 days?

Naegele’s rule assumes a 28-day cycle with ovulation around day 14. If your cycles run longer, you likely ovulated later, so your true due date is probably later than this estimate — and earlier if your cycles are short. Irregular cycles make LMP dating unreliable; a first-trimester scan handles that better.

How likely is my baby to arrive on the due date?

Not very — only about 1 in 20 babies arrive exactly on the estimated date. Most are born in the weeks around it, and anywhere from 37 to 42 weeks is considered term. Treat the date as the midpoint of a window, not an appointment.

Why did my doctor give me a different date?

Clinics usually confirm dating with an early ultrasound, which measures the embryo’s size and is more accurate than counting from your period — especially in the first trimester. When the scan disagrees with the LMP date, providers generally go with the scan. Use their date.

Can I calculate from my conception date instead?

Yes — a due date is roughly conception plus 266 days, since the standard 280 days includes about two weeks before ovulation. This tool works from the LMP because that’s the date most people know. If you conceived through IVF, your clinic dates the pregnancy from the transfer instead.

How many weeks pregnant am I?

Gestational age is counted from the first day of your last period, not from conception. So two weeks after your LMP you’re “2 weeks pregnant” even though conception is only just occurring, and on the estimated due date you’re exactly 40 weeks.

No — the date is calculated entirely in your browser and never sent to a server or stored.

Pregnancy Due Date Calculator

Estimate your due date from your last menstrual period (Naegele’s rule).

Runs in your browser — files never leave your device

First day of last period

Estimated due date

2024-10-07

Estimate via Naegele’s rule (LMP + 280 days). Consult your doctor.

How it works

This tool estimates your due date — formally the estimated date of delivery (EDD) — from the first day of your last menstrual period (LMP). Pregnancy is conventionally dated from the LMP rather than from conception, because the period start is a date most people actually know, while conception usually isn’t observable.

The calculation is Naegele’s rule, named after the 19th-century German obstetrician Franz Naegele and still the standard first estimate in maternity care: due date = first day of the LMP + 280 days (40 weeks). The rule assumes a typical 28-day cycle with ovulation around day 14, which is why the 280 days include roughly two weeks before conception even happened.

Worked example: if your last period started on January 10, 2026, adding 280 days gives an estimated due date of October 17, 2026. Because 280 days is exactly 40 weeks, the due date always lands on the same day of the week as your LMP — here, both are Saturdays — which is a quick way to sanity-check the result.

Read the date as the middle of a window, not a deadline. Anywhere from 37 to 42 weeks counts as term, most babies arrive in the weeks surrounding the estimate, and only about 1 in 20 arrive on the exact day. The date is still genuinely useful: it anchors your week-by-week progress, screening schedules, and appointment timing.

Limitations: the rule leans entirely on a remembered date and an assumed 28-day cycle. Longer, shorter, or irregular cycles shift real ovulation, and an uncertain LMP shifts everything. That’s why providers confirm dating with an early ultrasound and will re-date the pregnancy if the scan disagrees. This calculator is general guidance only — not medical advice; your doctor or midwife’s dating takes precedence.

Frequently asked questions

How is the due date estimated?: With Naegele’s rule: the calculator adds exactly 280 days — 40 weeks — to the first day of your last menstrual period. This is the same convention doctors and midwives use for initial dating before an ultrasound refines it.
What if my cycles are longer or shorter than 28 days?: Naegele’s rule assumes a 28-day cycle with ovulation around day 14. If your cycles run longer, you likely ovulated later, so your true due date is probably later than this estimate — and earlier if your cycles are short. Irregular cycles make LMP dating unreliable; a first-trimester scan handles that better.
How likely is my baby to arrive on the due date?: Not very — only about 1 in 20 babies arrive exactly on the estimated date. Most are born in the weeks around it, and anywhere from 37 to 42 weeks is considered term. Treat the date as the midpoint of a window, not an appointment.
Why did my doctor give me a different date?: Clinics usually confirm dating with an early ultrasound, which measures the embryo’s size and is more accurate than counting from your period — especially in the first trimester. When the scan disagrees with the LMP date, providers generally go with the scan. Use their date.
Can I calculate from my conception date instead?: Yes — a due date is roughly conception plus 266 days, since the standard 280 days includes about two weeks before ovulation. This tool works from the LMP because that’s the date most people know. If you conceived through IVF, your clinic dates the pregnancy from the transfer instead.
How many weeks pregnant am I?: Gestational age is counted from the first day of your last period, not from conception. So two weeks after your LMP you’re “2 weeks pregnant” even though conception is only just occurring, and on the estimated due date you’re exactly 40 weeks.
Is my data uploaded?: No — the date is calculated entirely in your browser and never sent to a server or stored.

Pregnancy Due Date Calculator

How it works

Frequently asked questions

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