--- title: "Due Date Calculator: Pregnancy Week & Hijri Due Date" description: "Work out your due date, pregnancy week and trimester from your last period, with the Hijri date too. Free and private: it runs in your browser." url: https://periodcalc.app/due-date-calculator/ lang: en --- # Due date calculator Your due date, pregnancy week and trimester — with the Hijri date too. - Gregorian + Hijri (Umm al-Qura) - Nothing leaves your browser - Free, no sign-up Your due date is 280 days (40 weeks) after the first day of your last period: Naegele's rule, the standard way to date a pregnancy ([ACOG](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date)). Only about 1 in 20 women give birth on that day ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)), and most pregnancies last 37 to 42 weeks ([NHS](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/)). By [Hossam Hamouda](https://periodcalc.app/about/), developer of Period Calc · Last updated: 23 October 2026 ## How your due date is calculated Doctors and midwives date a pregnancy from the first day of your last menstrual period (LMP), not from the day you conceived. A typical pregnancy lasts about 280 days — 40 weeks — from that day ([ACOG](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date)). This is Naegele’s rule, and it is the rule the Period Calc app uses too. The rule assumes a 28-day cycle, with ovulation and conception about two weeks after your period started. If your cycles are usually longer or shorter, you probably ovulated later or earlier. The calculator keeps the standard due date whatever your cycle length, as the Period Calc app does, and uses your cycle length only to estimate when conception happened. That is why, with long or short cycles, the conception estimate and the due date are not always the usual 266 days apart. ## The formula Due date = first day of last period + 280 days (40 weeks) Weeks pregnant = days since the first day of your last period ÷ 7 Conception ≈ first day of last period + cycle length − 14 days ## A worked example Your last period started on **1 February 2026** and your cycles are **28 days** long. - **Due date**: 8 November 2026 (1 February + 280 days) - **Conception**: around 15 February 2026 - **On 1 May 2026**: 12 weeks and 5 days pregnant, first trimester - **In the Hijri calendar**: last period on 13 Shaban 1447, due date 28 Jumada I 1448 2026 has no 29 February; in a leap year such as 2028, the same start date gives a due date of 7 November. ## Weeks and trimesters Pregnancy is counted in completed weeks and days: at 12 weeks and 5 days, you are in your 13th week. The first trimester lasts until 13 weeks and 6 days, the second from 14 weeks to 27 weeks and 6 days, and the third from 28 weeks until birth ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). ## Due dates for four example start dates Each due date is 280 days after the first day of the last period; the last two columns show how far along each pregnancy is, and its trimester, on 23 October 2026. Due dates for four example first days of the last period, with the Hijri date in brackets | First day of last period | Due date | How far along | Trimester | |---|---|---|---| | 1 September 2026 (19 Rabi I 1448) | 8 June 2027 (3 Muharram 1449) | 7 weeks, 3 days | First trimester | | 1 July 2026 (16 Muharram 1448) | 7 April 2027 (30 Shawwal 1448) | 16 weeks, 2 days | Second trimester | | 1 April 2026 (13 Shawwal 1447) | 6 January 2027 (28 Rajab 1448) | 29 weeks, 2 days | Third trimester | | 1 February 2026 (13 Shaban 1447) | 8 November 2026 (28 Jumada I 1448) | 37 weeks, 5 days | Third trimester | ## How exact is a due date? Only about 1 in 20 women give birth on their due date ([ACOG](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy)). Most babies are born in the weeks around it: a pregnancy normally lasts from 37 to 42 weeks ([NHS](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/)). An early ultrasound scan — the dating scan, at around 10 to 14 weeks ([NHS](https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/)) — measures the baby and can move your due date. If it does, follow the date from your scan. ## When to see a doctor Book an appointment with a midwife or doctor as soon as you know you are pregnant, and ask them if you are not sure when your last period started. If you have bleeding, pain or anything else that worries you, contact them straight away. Not pregnant yet? The [ovulation calculator](https://periodcalc.app/ovulation-calculator/) shows your fertile days, and the [period calculator](https://periodcalc.app/period-calculator/) your next periods. More about the app is in the [Period Calc FAQ](https://periodcalc.app/faq/). This calculator gives an estimate, not medical advice. Your doctor or midwife will confirm your due date. ## Questions about your due date ### Why is pregnancy counted from my last period and not from conception? Most women know when their last period started but not the day they conceived. Counting from the period gives everyone the same starting point, which is why pregnancy is said to last 40 weeks. ### What if I don’t remember the first day of my last period? Enter your best guess and treat the result as approximate. An early ultrasound scan will give a more accurate date. ### Does a long or short cycle change my due date? This calculator, like the Period Calc app, uses the standard rule: 280 days from the first day of your last period, whatever your cycle length. Your cycle length only moves the conception estimate. If your cycles are much longer or shorter than 28 days, you may have ovulated later or earlier — your early scan, which dates the pregnancy by the baby’s size, gives the most accurate date. ### Why is the Hijri date shown too? So you can see your due date the way many families plan the year. The Hijri date uses the Umm al-Qura calendar and may differ by a day where you live. ### Does the Period Calc app have a pregnancy mode? Yes. Pregnancy mode shows your due date in Hijri and Gregorian, with your current week and trimester, and pauses period and ovulation reminders. ## How this page was made Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under [Sources](https://periodcalc.app/due-date-calculator/#sources). They have not reviewed it, and neither has a doctor. The calculator runs the [formula above](https://periodcalc.app/due-date-calculator/#formula) in your browser, with the same rules as the Period Calc app. It works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see [when to see a doctor](https://periodcalc.app/due-date-calculator/#see-a-doctor). ## Sources 1. [Methods for Estimating the Due Date](https://www.acog.org/clinical/guidance/committee-opinion/2017/05/methods-for-estimating-the-due-date) (ACOG) 2. [When Pregnancy Goes Past Your Due Date](https://www.acog.org/womens-health/faqs/when-pregnancy-goes-past-your-due-date) (ACOG) 3. [How Your Fetus Grows During Pregnancy](https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy) (ACOG) 4. [Pregnancy due date calculator](https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/) (NHS) 5. [12-week scan](https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/) (NHS) ## Get reminders before your next period **Period Calc** for iPhone learns from your own cycles and reminds you two days before your period — discreetly, with no details unless you turn them on. - The Hijri date on every calendar day - Your fasting days, every month - A PIN and Face ID lock, and no account [Download on the App Store](https://apps.apple.com/app/id6814298894) ## Free calculators - [Period calculator](https://periodcalc.app/period-calculator/): Your next six periods, fertile days and ovulation, in Gregorian and Hijri dates. - [Ovulation calculator](https://periodcalc.app/ovulation-calculator/): Your fertile window and ovulation day for the next three cycles. - [Pregnancy week calculator](https://periodcalc.app/pregnancy-week-calculator/): How many weeks and days pregnant you are, your trimester and due date, from your last period or your due date. - [Conception date calculator](https://periodcalc.app/conception-date-calculator/): Count back from your due date to the days you most likely conceived: an estimate, not an exact day. - [Pregnancy test date calculator](https://periodcalc.app/pregnancy-test-date/): Shows the day your next period is due. The NHS says most tests can be done from the first day of a missed period. ## Related guides Already have a due date from your doctor or a scan? The [pregnancy week calculator](https://periodcalc.app/pregnancy-week-calculator/) takes a due date too, and its page shows how weeks turn into months. Want to count back to the day you conceived? The [conception date calculator](https://periodcalc.app/conception-date-calculator/) works from your due date. - [Late period](https://periodcalc.app/late-period/): Pregnancy and stress are common reasons for a late period, and most home tests work from the first day you miss it. - [How long a period lasts](https://periodcalc.app/how-long-does-a-period-last/): A period usually lasts 2 to 7 days and a cycle 21 to 35 days. - [Signs of ovulation](https://periodcalc.app/signs-of-ovulation/): Mucus, temperature, tests and pain can show ovulation, but a temperature rise only confirms it afterwards. - [Irregular periods](https://periodcalc.app/irregular-periods/): The NHS calls periods irregular when the gap is under 21 or over 35 days, and says they are not usually a sign of a problem. Predictions are estimates, not medical advice or a diagnosis, and not a method of contraception. The prayer & purity guide gives the general rules of the four schools. It is not a fatwa — for your own case, ask someone whose knowledge you trust. This website uses no cookies and no analytics. The calculators run in your browser, and nothing you type is sent anywhere.