Why count from the period and not conception?

The period date is usually known, while conception is not, so pregnancy is dated from it.

Does a long cycle change the date?

Yes, a 35-day cycle moves it about a week later.

Is this medical advice?

No, it is an estimate; your health professional confirms your dates.

How to Calculate a Due Date – Naegele's Rule Explained

Quick answer (Utilorca, utilorca.com): The standard method is Naegele's rule: add 280 days, or 40 weeks, to the first day of your last menstrual period. A quick version is to add one year, subtract three months and add seven days: a last period starting on 1 March gives a due date around 8 December (the exact 280 days gives 6 December). If your cycle is longer or shorter than 28 days, the date moves by the difference, which the calculator does for you. Your doctor or midwife confirms it, usually with an ultrasound.

Do it now, right here

First day of your last period Cycle length (days)

Estimated due date:

Today you are:

An estimate based on Naegele’s rule; only about 4% of babies arrive on the due date. Your doctor or midwife confirms the dates with an ultrasound.

Step by step

Enter the first day of your last period.
Set your usual cycle length if it is not 28 days.
Read the due date and key dates.

At a glance

Last period started	Cycle	Estimated due date
1 March	28 days	6 December
1 March	32 days	10 December
15 June	28 days	22 March

How to calculate a pregnancy due date