How is a pregnancy due date calculated?

By Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. The shortcut version is to subtract three months and add seven days plus one year. Both assume a 28-day cycle with ovulation on day 14, so this calculator also lets you correct for your own cycle length, which shifts the date by the difference between your cycle and 28 days.

How accurate is a due date?

As a prediction of the actual birthday, not very — only about 4% of babies arrive on their due date, and roughly half of first births happen after it. The useful figure is the range: around 80% of spontaneous births fall between 37 and 42 weeks. The due date is a reference point for scheduling scans and screening, not a deadline.

My cycle is not 28 days. Does that change the due date?

Yes, and this is where most calculators fall short. Ovulation happens roughly 14 days before your next period, not 14 days after the last one, so a 35-day cycle means you conceived about a week later than the standard formula assumes — pushing the due date a week back. Enter your average cycle length and the adjustment is applied automatically.

How do I calculate a due date after IVF?

Count from the transfer, not from a period. A day-3 embryo transfer already represents 17 days of gestation (14 days to notional ovulation plus 3 days of embryo growth), and a day-5 blastocyst represents 19 days. The due date is then 280 days from that derived start. Select the matching IVF option and the offset is handled for you.

Why does my ultrasound give a different due date than my period?

Because the scan measures the baby while the period only dates your cycle. A crown-rump length measured between 8 and 13 weeks dates a pregnancy to within about five days, so clinics follow ACOG guidance and replace the LMP-based date when the two disagree by more than seven days. Once that date is set in the first trimester, it is not changed by later scans.

What does '12 weeks 4 days' actually mean?

It is completed gestational age counted from the first day of your last period — 12 full weeks plus 4 days, written 12+4. Note that this is two weeks more than the age of the embryo itself, since conception happened around week 2. Being '12 weeks pregnant' means you are in your 13th week, which is why week counts in apps and books sometimes look off by one.

When does each trimester start and end?

First trimester: weeks 0 to 13+6. Second: weeks 14 to 27+6. Third: week 28 until birth. Different sources shift those boundaries by a week or so because there is no clinical consensus on the exact cut-off; this tool uses the 14/28 convention that ACOG and most hospital systems follow.

Is any of my data sent to a server?

No. The whole calculation is date arithmetic running in JavaScript on your device. Nothing is uploaded, stored or logged, and the page keeps working offline once it has loaded — which matters more than usual for a health query you may not want in a server log.

Pregnancy Due Date Calculator — Weeks, Trimester & Countdown

Calculate from

First day of your last period

Average cycle length

days

Estimated due date

—

0% of 40 weeks complete (0 days)

You are

0w 0d

Trimester

—

Conception (est.)

—

Full term from

—

week 37

Your trimesters

Trimester	Weeks	Dates	What happens
First trimester	Weeks 0–13	—	Organs form; the risk of miscarriage falls steadily through it.
Second trimester	Weeks 14–27	—	Nausea usually settles; the anatomy scan lands here.
Third trimester	Week 28 to birth	—	Weekly growth checks, then full term from week 39.

Milestones and appointment windows

Week	Date	Milestone
6	—	Heartbeat visible on an early scan
10	—	End of the embryonic period — now a fetus
12	—	Nuchal translucency scan and first-trimester screening
14	—	Second trimester begins
20	—	Anatomy scan — sex is usually visible
24	—	Threshold of viability outside the womb
28	—	Third trimester begins
28	—	Glucose tolerance test window closes
37	—	Early term — no longer counted as preterm
39	—	Full term
40	—	Estimated due date
42	—	Post-term — induction usually discussed before this

Dates assume a 40-week pregnancy from the derived start. Viability is reached around —, and the notional last-period date used for every calculation is —.

Which dating method to trust

All five methods produce a date. They do not produce equally good dates. If two disagree, the one with the smaller error wins.

Method	Typical error	Notes
Last menstrual period	±11 days	Reliable only with regular cycles. Naegele's rule assumes you ovulated on day 14, which roughly half of people do not.
First-trimester ultrasound (8–13 weeks)	±5 days	The crown-rump length is the single most accurate way to date a pregnancy. Clinics override the LMP date when the two differ by more than 7 days.
Second-trimester ultrasound	±10–14 days	Babies start growing at individual rates, so the measurement drifts. A date set in the first trimester is not revised later.
IVF transfer	±1 day	The only method where conception is known to the hour. No guessing about ovulation at all.

This calculator is a scheduling aid, not medical advice. Your midwife or doctor sets the official due date on your records, and that is the one your care plan follows.

Enter one date and get your estimated due date, how many weeks and days pregnant you are today, which trimester you are in, and the dates of every scan and screening window ahead. Works from your last period, a known conception date, a day-3 or day-5 IVF transfer, or an ultrasound measurement — and unlike most calculators it adjusts for cycles that are not 28 days long. Every calculation runs in your browser; no dates are sent anywhere.

How it works

1
Choose how you want to date the pregnancy Pick your last period if cycles are regular, a conception or ovulation date if you tracked it, the transfer date for IVF, or an ultrasound if you have a scan report. IVF and first-trimester ultrasound are the two most accurate options.
2
Enter the date and any extra detail For the last-period method, set your average cycle length — a 32-day cycle moves the due date four days later than the standard 28-day assumption. For an ultrasound, type the gestational age the sonographer wrote down, such as 9 weeks 3 days.
3
Read the due date, week and milestone dates The estimated due date appears with your current gestational age, trimester, percentage completed and a day countdown. Below that, every trimester boundary and appointment window is dated, with passed milestones ticked off.

Your data stays private

All processing happens entirely in your browser. No files, text, or data are ever sent to our servers. You can disconnect from the internet and this tool will still work.

Frequently asked questions

How is a pregnancy due date calculated?: By Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. The shortcut version is to subtract three months and add seven days plus one year. Both assume a 28-day cycle with ovulation on day 14, so this calculator also lets you correct for your own cycle length, which shifts the date by the difference between your cycle and 28 days.
How accurate is a due date?: As a prediction of the actual birthday, not very — only about 4% of babies arrive on their due date, and roughly half of first births happen after it. The useful figure is the range: around 80% of spontaneous births fall between 37 and 42 weeks. The due date is a reference point for scheduling scans and screening, not a deadline.
My cycle is not 28 days. Does that change the due date?: Yes, and this is where most calculators fall short. Ovulation happens roughly 14 days before your next period, not 14 days after the last one, so a 35-day cycle means you conceived about a week later than the standard formula assumes — pushing the due date a week back. Enter your average cycle length and the adjustment is applied automatically.
How do I calculate a due date after IVF?: Count from the transfer, not from a period. A day-3 embryo transfer already represents 17 days of gestation (14 days to notional ovulation plus 3 days of embryo growth), and a day-5 blastocyst represents 19 days. The due date is then 280 days from that derived start. Select the matching IVF option and the offset is handled for you.
Why does my ultrasound give a different due date than my period?: Because the scan measures the baby while the period only dates your cycle. A crown-rump length measured between 8 and 13 weeks dates a pregnancy to within about five days, so clinics follow ACOG guidance and replace the LMP-based date when the two disagree by more than seven days. Once that date is set in the first trimester, it is not changed by later scans.
What does '12 weeks 4 days' actually mean?: It is completed gestational age counted from the first day of your last period — 12 full weeks plus 4 days, written 12+4. Note that this is two weeks more than the age of the embryo itself, since conception happened around week 2. Being '12 weeks pregnant' means you are in your 13th week, which is why week counts in apps and books sometimes look off by one.
When does each trimester start and end?: First trimester: weeks 0 to 13+6. Second: weeks 14 to 27+6. Third: week 28 until birth. Different sources shift those boundaries by a week or so because there is no clinical consensus on the exact cut-off; this tool uses the 14/28 convention that ACOG and most hospital systems follow.
Is any of my data sent to a server?: No. The whole calculation is date arithmetic running in JavaScript on your device. Nothing is uploaded, stored or logged, and the page keeps working offline once it has loaded — which matters more than usual for a health query you may not want in a server log.

From the blog

How Due Dates Are Really Calculated (And Why Only 4% Arrive On Time) Naegele's rule, cycle-length corrections, IVF offsets and why an early ultrasound beats your calendar. Read the post →