Is the due date a guarantee?

No — only about 5% of babies arrive exactly on their due date. The clinically-meaningful range is anywhere between 37 and 42 weeks. The due date is a midpoint estimate.

What if my cycle is longer or shorter than 28 days?

Naegele's rule assumes a 28-day cycle and ovulation on day 14. If your cycle is 35 days, your true conception was likely 7 days later than the assumption — add 7 days to the calculated due date as a rough adjustment. Ultrasound dating supersedes this calculation in the first trimester.

Should I use LMP or conception?

LMP is the standard because most people remember their last period more reliably than the exact conception date. Use conception only if you tracked ovulation (basal body temperature, ovulation tests, IVF transfer date).

Does the calculator store my data?

No. Everything stays in your browser; no analytics events are sent with the input values.

Is this a substitute for an ultrasound?

No. First-trimester ultrasound dating is more accurate than LMP-based estimates because it measures the embryo directly. Always defer to your provider's dating once you have it.

Pregnancy Due Date Calculator

Naegele's rule + current gestational age + trimester. Estimates only — not a substitute for ultrasound.

Pregnancies last on average 280 days from the first day of the last menstrual period (LMP) — that’s Naegele’s rule, used throughout obstetrics since the 1830s. The calculator below applies it directly: enter your LMP (or conception date if you know it) and get a target due date, current gestational age in weeks + days, and trimester. Estimates only — your provider’s ultrasound dating in the first trimester is the gold standard.

First day of last period

Estimated due date

Thu, December 24, 2026

Gestational age today: 8 weeks 0 days · trimester 1

Estimates only. Naegele’s rule (LMP + 280 days) assumes a 28-day cycle and ovulation on day 14. Ultrasound dating from your provider is more accurate, especially in the first trimester.

How to use

Pick the method
LMP (last menstrual period) is the standard — the math assumes a 28-day cycle. Switch to conception date if you happen to know exactly when conception occurred (typical ovulation is ~day 14 of the cycle).
Enter the date
First day of your last period, or conception date. The calculator updates instantly.
Read the result
Big date is the estimated due date. Below it: weeks + days into the pregnancy today, plus which trimester that lands in.

Trimesters at a glance

Weeks (gestational)	Trimester
0–12	First
13–26	Second
27–40	Third
40+	Post-term

Frequently asked questions

Is the due date a guarantee?: No — only about 5% of babies arrive exactly on their due date. The clinically-meaningful range is anywhere between 37 and 42 weeks. The due date is a midpoint estimate.
What if my cycle is longer or shorter than 28 days?: Naegele's rule assumes a 28-day cycle and ovulation on day 14. If your cycle is 35 days, your true conception was likely 7 days later than the assumption — add 7 days to the calculated due date as a rough adjustment. Ultrasound dating supersedes this calculation in the first trimester.
Should I use LMP or conception?: LMP is the standard because most people remember their last period more reliably than the exact conception date. Use conception only if you tracked ovulation (basal body temperature, ovulation tests, IVF transfer date).
Does the calculator store my data?: No. Everything stays in your browser; no analytics events are sent with the input values.
Is this a substitute for an ultrasound?: No. First-trimester ultrasound dating is more accurate than LMP-based estimates because it measures the embryo directly. Always defer to your provider's dating once you have it.

About

Why 280 days?

Devised by German obstetrician Franz Karl Naegele in 1830. Add 7 days to the LMP, subtract 3 months, add 1 year — arithmetic that works out to LMP + 280 days. The number assumes a 28-day cycle and a 14-day luteal phase. Real pregnancy length is normally distributed with a standard deviation of about 13 days.

Trimester boundaries

There's no universal definition — the calculator uses the common 0-12 / 13-26 / 27-40 split. Some texts use 0-13 / 14-27 / 28-40. The difference is one week and doesn't change anything clinical.