How to Calculate a Pregnancy Due Date

How a due date is estimated

The usual way of estimating a due date is to start from the first day of the last menstrual period and add 40 weeks, which is 280 days. This method is simple and works without any tests. It assumes a typical cycle of about 28 days with ovulation around the middle.

Because the count begins at the last period and not at conception, a pregnancy is said to be two weeks along at about the time of conception. The 40 weeks therefore cover about 38 weeks of actual development.

This article explains the calculation, not medical care. Your midwife or doctor will give you a due date based on your own situation, and that is the one to use.

Naegele’s rule

A traditional shortcut is called Naegele’s rule. Take the first day of the last period, add one year, subtract three months and add seven days. It gives the same answer as adding 280 days in most cases.

For example, if the last period began on 10 January 2026, adding one year gives 10 January 2027, subtracting three months gives 10 October 2026 and adding seven days gives 17 October 2026. Adding 280 days to 10 January 2026 also gives 17 October 2026, a Saturday.

The two methods can differ by a day or so because months have different lengths, which is why many tools simply add 280 days.

Adding 40 weeks with the calculator

On the Add or subtract tab, enter the first day of the last period, choose Add and type 40 in the Weeks field, or 280 in the Days field. The result is the estimated due date, with its weekday.

You can also work out how far along a pregnancy is. On the days-between tab, enter the first day of the last period and today. The row Weeks and days gives the gestational age in the usual form, for example 10 weeks and 3 days.

If your cycle is longer or shorter than 28 days, the due date moves by the difference. A doctor may also adjust the date after an early ultrasound scan, which can measure the age of the baby more precisely.

A due date is an estimate

Only a small share of babies arrive on the estimated date. One medical source puts it at less than ten per cent. Most are born within about two weeks on either side, because the date is calculated from an assumed cycle and from an assumed day of conception.

Terms such as preterm, term and post-term describe the timing of birth. The World Health Organization calls a birth preterm if it happens before 37 completed weeks. Most healthy pregnancies last between 37 and 42 weeks.

So the date is best thought of as the middle of a window. Planning for the week before and the weeks after is more realistic than planning for one day.

Counting weeks and days correctly

Gestational age is written as completed weeks and days. A pregnancy at 12 weeks and 6 days has not yet reached 13 weeks. People often round up in conversation, but a medical record uses completed weeks.

A pregnancy of 40 weeks and 0 days is on the due date. Each additional day is written as 40 weeks and 1 day, and so on. The same counting method applies to a baby’s age in the first weeks, which is written as weeks and days or, later, in months.

For a quick conversion, divide the days since the first day of the last period by 7. For example, 74 days is 10 weeks and 4 days.

When the date is adjusted

An early ultrasound scan can give a different estimate, and clinicians often use it when the last period is uncertain or the cycle is irregular. If the scan and the period-based date differ by more than a short margin, the scan date is usually used.

Pregnancies after fertility treatment are dated from the known date of the procedure, because the day of conception is known. The calculator can help here too: add the right number of days to the transfer or insemination date, as advised by your clinic.

If you are unsure which date applies to you, ask. A calculator is a good way to understand the dates, and a professional is the right source for the date that guides your care.

Weeks and trimesters

Pregnancy is often described in trimesters as well as weeks. The first trimester runs to about the end of week 13, the second from week 14 to about week 27, and the third from week 28 to the birth. Because the boundaries are approximate and differ a little between sources, use the week count for anything exact.

Converting between the two is another job for a date calculator. Add 13 weeks to the first day of the last period to find when the first trimester ends, and 27 weeks for the end of the second.

Using the calculator for planning

Once you have the date, you can plan around it. The Day of the week tab shows the weekday of the due date, its ISO week and the days left in the year. The days-between tab shows how many days and workdays are left from today, which helps with leave planning.

Counting backwards is just as useful. Counting back from the due date also shows when a given week of pregnancy begins: subtract the number of weeks from the due date, and the calculator shows the date with its weekday.