How is the due date calculated?

The most common method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Only about 5% of babies are born on their exact due date.

What is gestational age?

Gestational age is measured from the first day of your last menstrual period, even though conception occurs about 2 weeks later. A full-term pregnancy is 40 weeks (280 days) of gestational age.

How accurate is the estimated due date?

Only about 4 to 5% of babies arrive on their exact due date; most are born within two weeks either side. An early ultrasound before 14 weeks dates a pregnancy more accurately than the last menstrual period, especially if your cycles are irregular.

How does an IVF transfer date change the due date calculation?

With IVF the embryo's age is known precisely, so the due date is counted from the transfer: a 5-day blastocyst transfer adds 261 days, while a 3-day embryo transfer adds 263 days. This is usually more accurate than dating from the last menstrual period.

What is Naegele's rule?

Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of your last menstrual period, assuming a regular 28-day cycle. It is the long-standing clinical standard but assumes day-14 ovulation, which may not match your cycle.

Can my due date change during pregnancy?

Yes. Providers often revise the due date after an early dating ultrasound if it differs from the menstrual-period estimate by more than about a week. Always rely on the date confirmed by your prenatal provider over any calculator.

Should I make firm plans around the exact due date?

Treat the due date as the centre of a likely delivery window, not a fixed appointment. For decisions about leave, travel or birth plans, confirm timing with your midwife or obstetrician, who can account for your individual pregnancy.

Pregnancy Due Date Calculator

How to use Pregnancy Due Date Calculator

The due date calculator estimates your expected delivery date (EDD) using four methods: last menstrual period (LMP) via Naegele's Rule, conception date, IVF 3-day embryo transfer or IVF 5-day blastocyst transfer. It also shows your current gestational age if you are already pregnant, trimester boundaries and six key prenatal milestones from the first prenatal visit to the 40-week mark.

Select your calculation method:LMP (most common), conception date, or IVF transfer date.
Enter the corresponding date using the date picker.
Your estimated due date, gestational age and trimester information appear immediately.
Review the milestone timeline for key prenatal dates.

Your data never leaves your device — 100% private processing.

Naegele's Rule and the 280-day standard

Naegele's Rule adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. Only about 4–5% of babies are born on their exact due date; most arrive within a two-week window on either side. The rule dates from 1812 but remains the worldwide clinical standard for dating pregnancies confirmed by ultrasound.

The three trimesters explained

Pregnancy is divided into three trimesters. The first trimester (weeks 1–12) covers implantation, organ formation and the highest risk of miscarriage. The second trimester (weeks 13–27) is typically the most comfortable, with anatomy scan around week 20. The third trimester (weeks 28–40) involves rapid fetal growth, lung maturation and preparation for birth. Premature birth is defined as delivery before 37 weeks.

Worked examples

From last menstrual period

Inputs: LMP = 1 Jan 2025

Result: +280 days → due ≈ 8 Oct 2025 (estimate)

From IVF blastocyst transfer

Inputs: 5-day transfer = 1 Feb 2025

Result: +261 days → due ≈ 20 Oct 2025 (estimate)

Due-date dating methods

Due-date dating methods
Method	Days added	Counted from
LMP (Naegele)	280 days	First day of last period
Conception date	266 days	Date of conception
IVF 3-day embryo	263 days	Transfer date
IVF 5-day blastocyst	261 days	Transfer date

Glossary

EDD: Estimated Due Date — the predicted date of delivery.
Gestational age: Pregnancy duration measured in weeks and days from the LMP.
LMP: Last Menstrual Period — the standard starting point for dating a pregnancy.
IVF: In vitro fertilisation — assisted reproduction where fertilisation occurs outside the body.
Trimester: One of three roughly equal stages of pregnancy, each approximately 13 weeks.

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Pregnancy Due Date Calculator

How to use Pregnancy Due Date Calculator

Naegele's Rule and the 280-day standard

The three trimesters explained

Worked examples

Due-date dating methods

Glossary

Related reading

Frequently Asked Questions

Why use Pregnancy Due Date Calculator?

Common use cases

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