Calculation method

Pregnancy calculation input

Format: YYYY-MM-DD, or pick it from the calendar
If your cycle is longer or shorter than 28 days, the date is adjusted automatically.
Format: YYYY-MM-DD, or pick it from the calendar
Format: YYYY-MM-DD, e.g. from your dating scan
Results

Enter a date on the left to see your pregnancy week, due date and baby size.

How does the pregnancy calculator work?

Pick whichever reference point you know – we'll work out the rest for you

Choose a method

Calculate from the date of your last period (the most common approach), your conception date, or a due date your midwife or doctor has already given you.

Enter the date

Fill in the relevant date. If you know your cycle length, add it too for a slightly more accurate result.

Check your results

You'll instantly see your pregnancy week, trimester, estimated due date and how big your baby is right now.

Track key dates

See your trimester boundaries, viability and full-term milestones, and the earliest point Statutory Maternity Leave can begin.

Why three different calculation methods?

Each pregnancy is calculated from a different reference point depending on what information you have – the result will be broadly similar in every case, but accuracy can vary slightly.

From your last period

The standard clinical method (Naegele's rule). 280 days are added to the first day of your last period. Best when you know the exact date.

From your conception date

A more precise method if you know the exact day of conception (for example, if you were tracking ovulation). Due date – 266 days (38 weeks) later.

From a dating scan

If your midwife or doctor has already given you a due date from an ultrasound scan, enter it directly – this is usually the most accurate option.

Frequently asked questions about pregnancy dating

Answers to the most common questions about pregnancy weeks and due dates

The most common method is Naegele's rule: 280 days (40 weeks) are added to the first day of your last period. If you know your conception date, 266 days (38 weeks) are added instead. This is a statistical estimate – actual birth can happen within a ±2 week window.
The exact day of conception is usually unknown, whereas the first day of the last period is a clearly identifiable reference point. Because of this, gestational age is medically counted from that date, even though actual conception happens roughly 2 weeks later.
Pregnancy is divided into three trimesters: 1st trimester – weeks 1–13, 2nd trimester – weeks 14–27, 3rd trimester – weeks 28–40+. Each trimester has its own typical stages of foetal development.
Statutory Maternity Leave can start any time from 11 weeks before your due date at the earliest, and starts automatically if you're off sick with a pregnancy-related illness in the four weeks before your due date, or if your baby is born early. Always check your exact entitlement with your employer or on GOV.UK.
Viability is generally considered to start around 24 weeks of pregnancy, and a baby is considered full term from 37 weeks. Birth up to 42 weeks still falls within the normal range.
Yes. All calculations happen in your browser – no data is ever sent to or stored on our servers.
No. This is a guidance tool based on statistical averages. Only a doctor or midwife, usually via an ultrasound scan, can confirm the exact stage of your pregnancy and due date.

Pregnancy dating – everything you need to know

A pregnancy calculator is a simple tool that helps you quickly work out which week of pregnancy you're in, which trimester you're in, and when your baby is likely due. The calculations rely on widely accepted clinical methods.

A standard pregnancy is calculated as 40 weeks (280 days) from the first day of your last period – this is known as Naegele's rule. If your menstrual cycle is longer or shorter than the typical 28 days, the calculator automatically adjusts the result.

Each week brings a different stage of foetal development – from a microscopic size in the earliest weeks to a fully formed baby close to birth. Our calculator compares each week to a familiar piece of fruit or veg, to make it easier to picture your baby's size.

It's worth remembering that only around 5% of babies are born exactly on their due date – most births happen within a ±1–2 week window around the estimated date, and that's entirely normal.

Besides the due date, the calculator also shows other key dates: trimester boundaries, the week you can usually find out your baby's sex, viability and full-term milestones, and the earliest point Statutory Maternity Leave can begin under UK rules.