Due Date Calculator

Last updated: April 2026

Method

Last period date

Cycle length

days

Estimated due date

8 October 2026

Gestational age today

24 weeks 2 days

Trimester

Second trimester

Estimated conception date

15 January 2026

Days remaining

110 days

Medical note

This calculator is an estimate only. Pregnancy dating should be confirmed by a qualified healthcare professional.

Related calculators:

Pregnancy Calculator
Pregnancy Conception Calculator
Ovulation Calculator
Period Calculator

How is a due date calculated?

The last menstrual period method estimates a due date by adding 280 days to the first day of the last period, with a cycle-length adjustment when needed.

Due date from conception

A conception-based estimate adds 266 days to the conception date, because this method starts around two weeks after the LMP dating method.

Due date from IVF

IVF dating adjusts for embryo age. A 3-day transfer adds 263 days, while a 5-day transfer adds 261 days.

Limitations

This is an estimate. Ultrasound and clinical dating may differ, and pregnancy dates should be confirmed by a qualified healthcare professional.

About this calculator

The Due Date Calculator estimates a pregnancy due date from the last menstrual period, conception date, IVF transfer date, or scan date where supported. It is useful for early planning, appointment timing, and understanding the approximate pregnancy timeline. A due date is an estimate, not a guaranteed delivery date. Many babies arrive before or after the calculated date, and clinical dating may be adjusted after ultrasound measurements.

Due date methodology

The common last menstrual period method estimates due date by adding 280 days, or 40 weeks, to the first day of the last period.

Estimated due date = first day of last period + 280 days
Pregnancy week = days since LMP / 7

How the calculator works

Choose the dating method you know best.
Enter the relevant date, such as last period or conception date.
The calculator estimates the due date and pregnancy timeline.
Use the result as a planning guide for appointments and milestones.

Worked examples

Last period method

Input: First day of last period 1 January

Calculation: 1 January + 280 days

Result: Estimated due date is around early October

Conception estimate

Input: Estimated conception date plus about 266 days

Result: The due date is estimated from the likely fertilisation date.

Irregular cycles

Input: Long or irregular cycle

Result: The estimate may be less accurate and ultrasound dating may be more useful.

How pregnancy due dates are estimated

A due date is an estimated date, not a guaranteed birth date. The most common calculation starts from the first day of the last menstrual period and adds 40 weeks. This method assumes a typical cycle and ovulation pattern, so it can be less accurate for irregular cycles.

Healthcare professionals may adjust dates after an ultrasound scan. Scan dating can be especially useful when the last period date is uncertain, cycles are irregular, or conception timing is unclear.

Dating methods explained

Different inputs can be used depending on what information is known. Each method has strengths and limitations.

Last menstrual period: This method counts from the first day of the last period and is widely used for early estimates.
Conception date: If conception timing is known, the estimate can be based on the shorter period from conception to birth.
IVF transfer date: IVF pregnancies may use embryo transfer date and embryo age, which can provide more precise dating.
Ultrasound date: Clinical scan measurements may be used by healthcare teams to confirm or revise an estimated due date.

Using the due date for planning

A due date can help plan appointments, maternity leave conversations, childcare, travel, and home preparation. It should not be used as a strict deadline. Many births happen before or after the estimated date, and clinical advice should guide medical decisions.

Why due dates change

Due dates can change after early scans because fetal measurements may suggest a different gestational age than the last period calculation. This is common and does not automatically mean anything is wrong. Healthcare teams use the dating method they consider most appropriate.

Pregnancy weeks and trimesters

Pregnancy is usually counted in weeks from the first day of the last menstrual period. This means the first two weeks of the count often occur before conception. Trimesters are broad stages used for planning care, screening, and development milestones.

When to seek professional guidance

Contact a midwife, GP, maternity unit, or qualified healthcare professional if you have pain, bleeding, severe symptoms, reduced movements later in pregnancy, uncertainty about dates, or any concern about your health. A calculator cannot assess pregnancy wellbeing.

Common mistakes and edge cases

Using the last day of the period instead of the first day.
Assuming ovulation happened on day 14 with irregular cycles.
Treating the due date as a fixed delivery date.
Ignoring updated dates from ultrasound scans.

Limitations

This calculator provides an estimate only. Pregnancy dates and health guidance should be confirmed with a qualified healthcare professional.

It does not diagnose pregnancy or assess pregnancy health.
Clinical teams may use scan measurements to adjust dates.

Frequently asked questions

How accurate is a due date?

It is an estimate. Many pregnancies deliver before or after the calculated date.

Why use 40 weeks?

The standard LMP method counts about 40 weeks from the first day of the last menstrual period.

What if my cycle is irregular?

Irregular cycles can make LMP dating less accurate, so a healthcare professional may rely more on scan dating.

Can this confirm pregnancy?

No. It only estimates dates after pregnancy is suspected or confirmed.

Should I change appointments based on this?

Use it for planning, but confirm timing with your midwife, GP, or maternity team.

Related calculators

Ovulation Calculator
Pregnancy Calculator
Conception Calculator
Pregnancy Weight Gain Calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.