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Pregnancy Due Date Calculator

Find your estimated due date, how many weeks pregnant you are and your key pregnancy milestones.

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Pregnancy Due Date Calculator Nothing is stored or sent

For general information only, not medical advice. Results are estimates and do not replace a check-up with a doctor or other qualified health professional.

What your result means

Your due date is the day you reach 40 weeks of pregnancy. It is an estimate, not a deadline. Most babies arrive in the weeks around it. The calculator also tells you how far along you are, which trimester you are in and how many days are left. Doctors use these terms from the American College of Obstetricians and Gynecologists (ACOG):

Stage Weeks of pregnancy
First trimester Up to 13 weeks 6 days
Second trimester 14 weeks 0 days to 27 weeks 6 days
Third trimester 28 weeks 0 days until birth
Preterm Before 37 weeks 0 days
Early term 37 weeks 0 days to 38 weeks 6 days
Full term 39 weeks 0 days to 40 weeks 6 days
Late term 41 weeks 0 days to 41 weeks 6 days
Post-term 42 weeks 0 days and beyond

Weeks are written as “weeks + days”. For example, 15 weeks 1 day means 15 full weeks and 1 extra day.

How the due date is calculated

Pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you conceived. This is because most people know when their period started but not the exact day they ovulated. In a 28-day cycle, ovulation and conception happen about two weeks after the period starts, so you are already “2 weeks pregnant” at conception.

From your last period (Naegele’s rule)

Due date = first day of last period + 280 days + (cycle length − 28) days

  • 280 days is 40 weeks, the average length of pregnancy counted from the last period.
  • Cycle length is the number of days from the start of one period to the start of the next. If your cycle is longer than 28 days, you probably ovulated later, so the due date moves later by the same number of days. A shorter cycle moves it earlier.

The classic paper version of Naegele’s rule is: add one year, take away three months and add seven days. Because months have different lengths, this can land a day or two away from adding exactly 280 days.

From your conception date

Due date = conception date + 266 days

266 days is 38 weeks, which is 280 days minus the two weeks between the period and conception. Use this if you know when you conceived, for example after IVF or a single known date of sex. After IVF, your clinic will give you a due date based on the embryo transfer date, so use theirs.

How to use the due date calculator

  1. Choose your method: first day of your last period, or conception date.
  2. Enter the date.
  3. If you chose the period method, enter your average cycle length (21 to 45 days). Leave it at 28 if you are not sure.
  4. Read your due date, how far along you are today, your trimester, days to go and your milestone dates.

Worked example

The first day of your last period was Sunday 1 March 2026, and your cycle is 28 days.

  • Due date: 1 March 2026 + 280 days = Sunday 6 December 2026.
  • Second trimester starts (14 weeks): 7 June 2026.
  • Anatomy scan window (18 weeks 0 days to 22 weeks 6 days): 5 July to 8 August 2026.
  • Third trimester starts (28 weeks): 13 September 2026.
  • Full term (39 weeks): 29 November 2026.

On 15 June 2026, 106 days have passed since your period started. That is 15 weeks 1 day, so you are in the second trimester, with 174 days to go.

If your cycle were 32 days instead, your due date would move 4 days later, to Thursday 10 December 2026. If you knew you conceived on 15 March 2026, the conception method gives 15 March + 266 days = 6 December 2026, the same date as the 28-day period method.

How accurate is your due date?

Only about 4% of babies are born on the exact due date, and only about 70% arrive within 10 days of it, even when the date comes from an ultrasound (Jukic and colleagues, Human Reproduction 2013 (opens in new tab)). The same NIH study tracked pregnancies from a known ovulation date and found natural pregnancy length still varied by 37 days. A due date is best thought of as the middle of a window, not a fixed day. The NHS (opens in new tab) puts a normal pregnancy at 37 to 42 weeks.

Why an ultrasound can change your due date

An ultrasound in the first trimester is the most accurate way to date a pregnancy. The sonographer measures the baby from head to bottom, called the crown-rump length (CRL). Babies grow at a very similar rate early on, so this measurement gives a reliable age.

ACOG and the American Institute of Ultrasound in Medicine (AIUM) set how big the gap must be before a doctor switches to the scan date. The allowed gap widens as pregnancy goes on, because babies grow at different rates later (AIUM summary of ACOG Committee Opinion 700 (opens in new tab)):

  • Up to 8 weeks 6 days: change the date if the scan differs by more than 5 days.
  • 9 weeks to about 16 weeks: more than 7 days.
  • 16 weeks to 21 weeks 6 days: more than 10 days.
  • 22 weeks to 27 weeks 6 days: more than 14 days.
  • 28 weeks or later: more than 21 days.

ACOG also says a pregnancy without a scan that confirms or revises the due date before 22 weeks should be treated as suboptimally dated (Obstetrics and Gynecology, 2017 (opens in new tab)). Once your doctor sets a due date, it is rarely changed again.

If your periods are irregular

The cycle length setting helps if your cycles are regular but longer or shorter than 28 days. If your periods are very irregular, if you do not remember your last period, or if you got pregnant soon after stopping the pill or while breastfeeding, the period method may be wrong by a week or more. An early dating scan will give a better date.

Scans and milestones

Two scans matter most for dates. The dating scan, often combined with the nuchal translucency (NT) measurement for Down’s syndrome screening, is done at about 10 to 14 weeks (NHS (opens in new tab)). The anomaly scan, which Indian clinics often call a level II scan, is usually done at 18 to 21 weeks and sometimes up to 23 weeks (NHS (opens in new tab)). The calculator’s anatomy scan window covers 18 to 22 weeks. Your doctor will decide which scans you need and when.

In India, the PCPNDT Act bans prenatal sex determination, so no scan centre can tell you the sex of your baby (National Health Mission, Kerala (opens in new tab)).

You do not need to eat for two

ICMR-NIN advises pregnant women to eat about 350 kcal a day more than their usual need, adjusted for their weight and weight gain (ICMR-NIN, 2020 (opens in new tab)). That is about one extra small meal or snack a day, not a second plate of everything. Breastfeeding raises the need further, by about 600 kcal a day in the first six months.

Limitations

  • The calculator gives an estimate based on averages. It cannot predict when labour will start.
  • It does not replace a dating scan or your doctor’s or midwife’s due date.
  • It is less reliable with irregular cycles, an unknown period date or recent hormonal contraception.
  • Twins and other multiple pregnancies are often born earlier than the due date.

Book your first antenatal visit as soon as you know you are pregnant. Get medical help straight away if you have heavy bleeding, severe tummy pain, a severe headache with blurred vision, fluid leaking from the vagina, or if your baby moves less than usual later in pregnancy.

Key facts

  • A pregnancy is about 280 days (40 weeks) long when counted from the first day of the last menstrual period (ACOG (opens in new tab)).
  • An ultrasound measurement of the baby in the first trimester is the most accurate way to set or confirm the due date (ACOG (opens in new tab)).
  • Full term is 39 weeks 0 days to 40 weeks 6 days; early term is 37 weeks 0 days to 38 weeks 6 days (ACOG (opens in new tab)).
  • A baby born alive before 37 weeks of pregnancy is preterm (WHO (opens in new tab)).
  • Only about 4% of babies are born at exactly 280 days, and about 70% within 10 days of the due date (Human Reproduction, 2013 (opens in new tab)).
  • Pregnant women need about 350 kcal a day extra, according to India’s 2020 nutrient requirements (ICMR-NIN (opens in new tab)).
  • The dating scan offered on the NHS is usually done between 10 and 14 weeks of pregnancy (NHS (opens in new tab)).
  • Conception usually happens about two weeks after the first day of the last period, so the due date is about 266 days after conception (ACOG (opens in new tab)).

FAQ

Pregnancy Due Date Calculator FAQs

How is my due date calculated?

Your due date is calculated by adding 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves by the difference. If you know your conception date, add 266 days instead. An early ultrasound may then confirm or adjust the date.

How accurate is a due date?

A due date is an estimate. Only about 4% of babies arrive on the exact day, and about 70% within 10 days of it, according to an NIH study published in 2013. Most births fall between 37 and 42 weeks. A due date from a first-trimester ultrasound is more accurate than one based only on your last period, especially if your cycles are irregular.

What if my periods are irregular?

If your periods are irregular, a due date from your last period may be off by a week or more, because ovulation timing is unknown. Enter your average cycle length if you know it. If your cycles vary a lot, an early dating ultrasound is the best way to find your due date, and your doctor will use that.

Can an ultrasound change my due date?

Yes, an ultrasound can change your due date, mainly if it is done early. ACOG advises using the scan date when it differs from the period-based date by more than 5 days before 9 weeks, or more than 7 days up to about 16 weeks. Later scans need a bigger gap. Once the date is set, later scans do not usually change it.

How many weeks pregnant am I?

You are as many weeks pregnant as the number of full weeks since the first day of your last period. For example, 106 days after your period started is 15 weeks 1 day. This count includes about two weeks before conception. Enter your period date in the calculator to see your exact weeks and days today.

When does each trimester start?

The first trimester starts on the first day of your last period and runs to 13 weeks 6 days. The second trimester starts at 14 weeks 0 days and ends at 27 weeks 6 days. The third trimester starts at 28 weeks 0 days and lasts until birth. The calculator shows the exact date each one starts for you.

How many babies are born on their due date?

Only about 4 in 100 babies are born on the exact due date. Most are born in the weeks around it, between 37 and 42 weeks, and even pregnancies dated from a known ovulation day vary in length by over a month. This is normal. Your due date simply marks the middle of the period when your baby is most likely to arrive.

Can I calculate my due date from the conception date?

Yes, you can calculate your due date by adding 266 days (38 weeks) to your conception date. This gives the same result as the period method in a 28-day cycle, because conception usually happens about two weeks after the period starts. After IVF, use the due date your fertility clinic gives you, as it is based on the embryo transfer date.