How to work out your due date, step by step

One of the first questions after a positive pregnancy test is when the baby is due. The answer is an estimate, but it is a useful one: it sets the timing of your scans, your antenatal appointments and your plans at work. The calculation is straightforward once you know what the 40 weeks are counted from, and it can be adjusted if your cycle is longer or shorter than average. This guide shows the three main ways to work out a due date, with dated examples you can check, explains how scans can change the date, and covers the key dates for time off work in the UK.

By Valtrinity. Updated 18 October 2026.

How pregnancy is counted

Pregnancy is dated from the first day of your last menstrual period, not from the day you conceived. A full-length pregnancy is counted as 40 weeks, which is 280 days, from that date. Because ovulation and conception usually happen about two weeks after your period starts, you are counted as being two weeks pregnant at the moment of conception.

That can seem odd, but it is the method used by health services everywhere, because the first day of your last period is usually easier to remember than the day of conception. It is also why a pregnancy described as 12 weeks has been under way for about 10 weeks since conception.

The NHS says that most babies are born between 37 and 42 weeks. The due date is a point in the middle of that window, not a deadline, and only a minority of babies arrive on the day.

Method 1: add 280 days to your last period

Take the first day of your last period and add 280 days. Suppose your last period started on 10 January 2027. Counting forward 280 days brings you to 17 October 2027. That is your estimated due date.

A shortcut that gives nearly the same result is known as Naegele's rule: take the first day of your last period, add seven days, subtract three months, and add one year. For 10 January 2027, add seven days to get 17 January, subtract three months to get 17 October, and the year is 2027. The answer matches.

The rule is an approximation, because months differ in length, so it can be a day out for some dates. Counting 280 days exactly is more reliable, and a calculator does it in a moment.

Method 2: adjust for your cycle length

The 280-day count assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation, and so conception, happens at a different point, and the due date shifts with it.

The adjustment is simple: add the number of days by which your cycle is longer than 28, or subtract the number of days by which it is shorter. If your cycle is usually 32 days, add 4 days. If it is 25 days, subtract 3 days.

Using the earlier example, a last period starting on 10 January 2027 with a 28-day cycle gives 17 October 2027. With a 35-day cycle you would add 7 days, which gives 24 October 2027. With a 24-day cycle you would subtract 4 days, which gives 13 October 2027. If your cycles are irregular, the calculation is less reliable and a scan is the better guide.

Method 3: from a conception or ovulation date

If you know when you conceived, for example through fertility tracking or a single occasion, add 266 days, which is 38 weeks. This is 280 days minus the 14 days between the start of your period and ovulation.

For a conception date of 24 January 2027, adding 266 days gives 17 October 2027. Notice that it matches the first example, because a last period on 10 January and ovulation on 24 January are 14 days apart.

If you had fertility treatment, the clinic will usually give you the due date based on the transfer or insemination date, taking into account the age of the embryo. Use that date in preference to a calendar calculation.

Why a scan can change the date

The dating scan, usually offered between about 10 and 14 weeks, measures the baby and compares it with expected sizes for each week. If the measurement differs from your calculated date by more than a few days, the sonographer or midwife may change your due date to the scan date.

Early in pregnancy, babies are very similar in size, so a scan is accurate to within a few days at that stage. Later in pregnancy, babies vary more in size, so later scans measure growth and are not used to change the date.

If your date is moved, it does not mean anything is wrong. It usually means that you ovulated earlier or later than the 28-day pattern assumes. Use the date your midwife gives you for appointments and for telling your employer.

Week-by-week landmarks

Once you have a due date, you can count back to find the milestones. The end of the first trimester is at 13 weeks. The mid-pregnancy scan is usually offered between 18 and 21 weeks. The second trimester runs from 14 to 27 weeks, and the third from 28 weeks onwards.

The simplest way is to add the week count, in days, to the first day of your last period. With a last period starting on 10 January 2027, 12 weeks is 84 days, which gives 4 April 2027. At 20 weeks, which is 140 days, the date is 30 May 2027. At 37 weeks, which is 259 days, it is 26 September 2027, and at 40 weeks it is 17 October 2027.

Babies born before 37 weeks are premature, and those born from 37 weeks are considered to be full term. Your midwife will talk to you about the options if you pass your due date, usually around 41 to 42 weeks.

Key dates for work in the UK

For statutory maternity leave, GOV.UK says you can start your leave from 11 weeks before the week your baby is due. The qualifying week is the 15th week before the expected week of childbirth, and you normally need to tell your employer that you are pregnant, the date of the expected week of childbirth and when you want your leave to start by the end of that qualifying week.

Statutory maternity leave lasts up to 52 weeks. Eligibility for pay depends on your employment and earnings, so check the current rules and rates on GOV.UK, because they change each April.

If you want to plan, work backwards from the due date. The expected week of childbirth is the week, running Sunday to Saturday, that contains the due date. For a due date of Sunday 17 October 2027 it starts that day. The 15th week before it begins on Sunday 4 July 2027 and ends on Saturday 10 July, and the earliest leave start is 11 weeks before, on 1 August 2027. Confirm these dates with your employer and the official guidance, because the exact week definition matters.

Tools mentioned in this guide

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