Due Date Calculator

Written by Cynthia Obinatu, community and editorial lead. Checked against published guidance from the National Health Service, the World Health Organization and peer-reviewed research. Not yet reviewed by a clinician. Educational, not medical advice.

How this due date calculator works

It takes the first day of your last period, adds 280 days (40 weeks) and adjusts for your cycle length. The standard method assumes a 28-day cycle with ovulation on day 14. If your cycle is 32 days, you probably ovulate about four days later, so the calculator moves your due date four days later too. The NHS calculator also asks you to adjust the cycle length, because your cycle "may not be exactly 28 days".

Pregnancy is counted from the first day of your last period. On a 28-day cycle that means you are already about two weeks pregnant on the day an egg is fertilised. If you want to work back from the day you ovulated, the ovulation calculator estimates it. The NHS due date calculator says that if you do not know the date or are unsure of it, you should speak to a midwife or doctor, and the same applies in Nigeria. A midwife or doctor at your first antenatal visit can arrange a scan to date the pregnancy.

What your due date result means

Read your due date as the middle of a range. The NHS says pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period, so your baby can arrive well before or after the date on the screen and everything can still be normal. The calculator also shows how many weeks and days pregnant you are today and which trimester you are in. It counts completed weeks, with the second trimester starting at 13 weeks and the third at 28. For what happens in each stage, see the trimester by trimester guide.

How many days am I pregnant from my last period?

Count the days from the first day of your last period to today. If your period started on 1 September and today is 1 October, you are 30 days pregnant. That is 4 weeks and 2 days. To turn days into weeks, divide by 7. The whole number is your weeks and the remainder is your extra days. The calculator does this for you and shows the answer under your due date.

How do I calculate my due date manually?

Take the first day of your last period and add 280 days. A quicker way on a calendar is to add seven days to that date, then count back three months. If your last period started on 1 March, that gives 8 December. The 280-day count gives 6 December, and the two methods differ by a day or two. If your cycle is longer or shorter than 28 days, add or subtract the difference. A 32-day cycle adds four days and a 25-day cycle takes away three.

Is due date 40 or 42 weeks?

Your due date is at 40 weeks. That is the date the calculator gives you, and 42 weeks is the upper end of the normal range. The NHS says a pregnancy normally lasts from 37 to 42 weeks. The World Health Organization calls a baby preterm if it is born before 37 completed weeks. Going past 40 weeks falls inside the normal range, and your midwife or doctor will tell you what to expect and when they want to see you.

A study of different ways to date pregnancy in Archives of Gynecology and Obstetrics found the average length of pregnancy counted from the last period was close to the standard figure, at 279 days. Individual pregnancies vary around it.

When will I be 37 weeks pregnant?

You reach 37 weeks 259 days after the first day of your last period. That is three weeks before a 40-week due date. If your last period started on 1 March, you reach 37 weeks on 15 November in a standard year, and your due date is 6 December. Count three weeks back from your own due date to find yours.

The date matters because 37 weeks is where the WHO stops calling a birth preterm. Babies born from 37 weeks are not preterm by that definition, and the WHO fact sheet treats anything earlier as preterm.

What is the most accurate due date calculator?

None of them beats a dating scan. Every calculator, including this one and the one on the NHS site, does the same arithmetic on the date you enter, so the accuracy depends on that date. The NHS says the 12-week scan can estimate how many weeks pregnant you are more accurately. Choose a calculator that asks for your cycle length, as this one does, and then treat your scan date as the real one.

People often misremember the date of their last period. A study published through PubMed Central noted that certainty about the last period has been reported as low as 32 percent, and that dates such as the 15th of the month turn up about two and a half times more often than chance would predict. Write the date down the day your period starts.

Which due date is correct in pregnancy?

If the date from your last period and the date from your scan disagree, your midwife or doctor will tell you which one they are using. The NHS describes the scan as the more accurate estimate, so when the two differ by more than a few days, the scan date is usually the one on your records. Use that date for everything that follows, including the calculator's weeks.

Is 36 weeks exactly 9 months?

No. Thirty-six weeks is about 8.3 calendar months. Most charts count it as the start of the ninth month, and your due date sits at 40 weeks. The next section explains how weeks turn into months.

How many months pregnant am I? Weeks to months

To convert weeks to months, divide your weeks by 4.35, the number of weeks in an average calendar month. Thirty weeks gives 6.9, so 30 weeks pregnant is about 7 months. Twenty-four weeks is about 5.5 months, and most charts call that month 6. Forty weeks is about 9.2 months. That is why pregnancy is called nine months even though it runs a little longer.

Sources do not agree on the month boundaries, because a month is not a whole number of weeks. This page uses two conventions. The calendar column divides weeks by 4.35. The pregnancy month column groups the 40 weeks into nine months of 4, 4, 5, 4, 4, 5, 4, 5 and 5 weeks, a common way to lay out a chart. Other charts move a week or two at the edges, so a friend's app may say month 8 when this table says 7. The weeks are the same in every source.

Doctors and midwives use weeks. The NHS week by week guide is organised in weeks, with the first trimester at weeks 4 to 12, the second at 13 to 27 and the third at 28 to 41. Use weeks when you book at an antenatal clinic in Nigeria. Months are fine for telling family how far along you are. The week by week tracker shows what is happening at each week.

Common questions

Where this comes from

• Pregnancy due date calculator. National Health Service.
• Your pregnancy week by week. National Health Service.
• Preterm birth fact sheet. World Health Organization.
• Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing. Archives of Gynecology and Obstetrics, via PubMed Central, 2016.

This page has not yet been reviewed by a clinician. It stands on the published guidance above, and we say so plainly rather than implying a review that has not happened. Read it alongside our medical policy.