How accurate is a due date?

It is an estimate. Only about 4% of babies are born on their due date, and most arrive in the two weeks before or after it. An early ultrasound scan, usually between 8 and 14 weeks, gives the most reliable date and may move your due date by a few days.

Why is pregnancy counted from my last period and not from conception?

The first day of the last period is a date most people know, while the exact day of conception is rarely known. By convention, pregnancy is counted from that first day, so at conception you are already about two weeks pregnant on paper.

My cycle is not 28 days. Does that change my due date?

Yes. Ovulation usually happens about 14 days before the next period, so a longer cycle means later ovulation and a later due date. The calculator moves the date by one day for every day your cycle is longer or shorter than 28 days.

How is the due date calculated after IVF?

With IVF the age of the embryo is known exactly. A day-5 embryo transferred today was, on paper, conceived 5 days ago, so the due date is 261 days after the transfer. For a day-3 embryo it is 263 days.

Can my due date change?

It often does after the first scan. If the scan and your period-based date differ by more than about a week, your midwife or doctor will usually go by the scan. After that first adjustment, the date normally stays the same.

Due date calculator: when is my baby due?

How the due date is calculated

The calculator uses Naegele's rule, the method midwives and doctors have used for nearly two centuries. It takes the first day of your last period and adds 280 days, which is 40 weeks. That date is your estimated due date.

The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation usually moves too, so the calculator shifts the due date by the difference. With a 32-day cycle, for example, the due date moves 4 days later.

If you know the day you conceived, the calculator counts 266 days (38 weeks) from that day instead. After IVF it uses the transfer date and the age of the embryo, because the timing of fertilisation is known exactly: 261 days after a day-5 transfer, 263 days after a day-3 transfer.

What the weeks and trimesters mean

Pregnancy is counted in completed weeks and days from the first day of the last period, written for example as 14 weeks and 6 days. That means you are already counted as two weeks pregnant on the day you conceive.

The 40 weeks are split into three trimesters:

First trimester: weeks 0 to 13. The baby's organs form and most early symptoms, such as nausea and tiredness, are at their strongest.
Second trimester: weeks 14 to 27. Many people feel more energetic, the bump starts to show and the first movements are felt, often between 16 and 22 weeks.
Third trimester: week 28 until birth. The baby gains most of its weight and moves into position for birth.

A pregnancy is called full term from 39 weeks, early term from 37 weeks and late term from 41 weeks.

Why your baby probably won't come on the due date

A due date is the middle of a window, not an appointment. Only about 1 baby in 25 is born on the day itself. Most are born between 37 and 42 weeks, and first babies tend to arrive a little later than second or third babies.

That is why many parents find it more useful to think of a due month, or of the two weeks either side of the date, when planning leave or family visits.

When the date changes

The first ultrasound scan measures the baby, usually from crown to rump, and compares the size with what is typical for each day of pregnancy. Early in pregnancy babies grow at very similar rates, so this measurement is accurate to within about 5 to 7 days.

If the scan date and the period-based date differ by more than that, the scan date is usually kept. This is common when cycles are irregular, when the last period date is uncertain, or after recently stopping hormonal contraception.

Using your due date to plan

Once you have an estimated date, a few others follow from it. The first scan is usually offered between 11 and 14 weeks, often together with screening tests, and the detailed anomaly scan around 20 weeks. Many employers and parental leave schemes ask for the due date in writing, usually from your midwife or doctor, some weeks before the birth. If you are planning leave, a hospital bag or visits from family, it helps to plan around the whole window from 37 to 42 weeks rather than one day.

If you know your due date but not the start of your last period, you can work backwards: the first day of the last period is 280 days before the due date. The pregnancy week calculator does this for you and shows how far along you are today.

Limits of this calculator

This tool gives the same estimate a pregnancy wheel would. It cannot account for irregular cycles, bleeding in early pregnancy that was mistaken for a period, or medical conditions. Your midwife or doctor will confirm your due date at your first appointment and scan.