Due Date Calculator

Calculate your estimated due date from your last period, ultrasound, conception date, or IVF transfer. Track pregnancy milestones and trimester progress.

Free to use. Runs in your browser.

Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle.

Enter your information below for a personalized due date calculation.

Calculate Your Due Date

First Day of Last Period

Cycle Length (days)

Average cycle is 28 days. Range: 22-44 days.

Educational Estimates Only: These calculations are approximations based on population-level formulas and may not accurately reflect your individual health status. Body composition varies significantly due to factors like muscle mass, bone density, hydration, and ethnicity. This is not medical advice. Always consult a qualified healthcare provider for personalized assessment, diagnosis, or treatment decisions.

How Due Dates Are Actually Calculated

The standard method for calculating a due date is Naegele's rule, published in 1812: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. This gives you a date 280 days (40 weeks) from your LMP.

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, ovulation happens at a different time, and your due date shifts accordingly. A 35-day cycle means ovulation around day 21, adding a full week to the standard calculation.

This is why cycle length matters. This calculator adjusts for non-standard cycles, and also accepts ultrasound dating, known conception dates, and IVF transfer dates, each with different accuracy.

Accuracy of Different Dating Methods

Method	Accuracy	When to Use
First-trimester ultrasound	± 5 to 7 days	Most accurate method, gold standard before 13 weeks
IVF / known conception	± 1 to 3 days	Most precise, since fertilisation date is known exactly
LMP (regular 28-day cycle)	± 2 weeks	Good estimate if you know your LMP and have regular cycles
LMP (irregular cycles)	± 3 to 4 weeks	Least reliable, confirm with early ultrasound
Second-trimester ultrasound	± 1 to 2 weeks	Less accurate than first trimester but still useful

What this means for you: If your LMP-based due date and your first ultrasound due date differ by more than 7 days, most clinicians will go with the ultrasound date. The earlier the ultrasound, the more accurate it is, embryos grow at very predictable rates in the first trimester.

When Do Babies Actually Arrive?

Only about 5% of babies are born on their exact due date. Here's the actual distribution:

Gestational Age	Classification	Percentage of Births
Before 37 weeks	Preterm	~10%
37 to 38 weeks	Early term	~25%
39 to 40 weeks	Full term	~50%
41 weeks	Late term	~10%
42+ weeks	Post-term	~5%

First-time mothers tend to deliver slightly later (average 40 weeks + 5 days). Second and subsequent pregnancies tend to be a few days earlier. These are averages, individual variation is wide.

Key Milestones Before Your Due Date

Week	Milestone	Why It Matters
12	Dating scan	Most accurate due date estimate, can shift your date by up to 2 weeks
20	Anomaly scan	Detailed check of baby's development; often when parents learn the sex
28	Third trimester begins	Appointments increase to fortnightly; start thinking about birth plan
34	Maternity leave earliest start	You can start mat leave from week 29, but most wait until 34-36
36	Hospital bag packed	Baby could arrive any time from now, don't leave it later
37	Full term	Baby is considered full term; lungs are mature
41	Induction offered	NHS typically offers induction at 41-42 weeks

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How to use this tool

Select your calculation method (Last Period, Ultrasound, Conception, or IVF)

Enter the required date and any additional information

Click 'Calculate Due Date' to see results

Common uses

Calculating your estimated due date
Adjusting for irregular cycle lengths
Comparing LMP and ultrasound dating
Planning maternity leave timing
Tracking pregnancy milestones

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Frequently Asked Questions

How is my due date calculated?

Due dates are calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's rule. If your cycle isn't 28 days, the calculation adjusts, a 35-day cycle shifts the due date a full week later.

How accurate are due date calculations?

Only about 5% of babies arrive on the exact due date. Most births occur within a 5-week window from 37 to 42 weeks. First-trimester ultrasounds are accurate to within 5 to 7 days, while LMP-based dates can be off by 2 to 3 weeks for irregular cycles.

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

Cycle length matters because it shifts when you ovulate. In a 35-day cycle, ovulation occurs around day 21 instead of day 14, adding a full week to the standard calculation. This calculator adjusts for your actual cycle length.

When is the best time to have a dating ultrasound?

First-trimester ultrasounds (before 13 weeks) are most accurate because embryos grow at very predictable rates in early pregnancy. After 13 weeks, natural size variation makes dating progressively less precise.

Why is my ultrasound due date different from my LMP due date?

If they differ by more than 7 days, the ultrasound date is usually more accurate. LMP dating assumes you ovulated on day 14 of a 28-day cycle, which many women don't. The earlier the ultrasound, the more reliable it is.

What percentage of babies are born on their due date?

Only about 5%. About 50% are born within a week of their due date, and roughly 90% arrive between weeks 37 and 42. First-time mothers tend to deliver slightly later on average (40 weeks + 5 days).

What happens if I go past my due date?

Going past your due date is common, about 15% of pregnancies continue past 41 weeks. Most hospitals offer a membrane sweep around 40 to 41 weeks and recommend induction between 41 and 42 weeks, as risks increase slightly after that point.

Does cycle length really affect my due date?

Yes, significantly. The luteal phase (ovulation to period) is fairly constant at 12 to 14 days, but the follicular phase varies. A 35-day cycle means ovulation around day 21, adding 7 days to the due date compared to a standard 28-day calculation.

How is an IVF due date calculated?

IVF due dates are the most precise because fertilisation date is known exactly. For a day-5 blastocyst transfer, the due date is 261 days from transfer. This removes all guesswork about ovulation timing.

Can my due date change during pregnancy?

Clinicians may adjust your due date after a first-trimester ultrasound if it differs from the LMP date by more than 7 days. After the first trimester, the due date is typically not changed because later ultrasounds have wider margins of error.

Is the first baby usually late?

Statistically, first-time mothers deliver about 5 days after their due date on average. But this is an average, plenty of first babies arrive early. Second and subsequent babies tend to arrive slightly sooner.

What is Naegele's rule?

Published in 1812, Naegele's rule is the standard method: take the first day of your last period, add one year, subtract three months, and add seven days. This gives 280 days from LMP, assuming a 28-day cycle with ovulation on day 14.

Results are for general informational purposes only and should be checked before use. They are not professional advice. See our Disclaimer and Terms of Service.