Due Date Calculator
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: A due date calculator estimates when you will reach 40 weeks of pregnancy. Enter the first day of your last period and it adds 280 days. Enter a conception date and it adds 266 days. For IVF it uses your transfer date and embryo age. Your provider may adjust the date after an early ultrasound.
Use the due date calculator above, then read on to understand where the number comes from, how far to trust it and what to do next.
How the due date calculator works
Every pregnancy calculator uses the same basic idea: pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you conceived. That is why a "40-week" pregnancy only includes about 38 weeks of your baby's actual growth. The first two weeks cover your period and the days before ovulation.
| What you enter | What the calculator does | Example |
|---|---|---|
| First day of last period (28-day cycle) | Adds 280 days | LMP 1 March 2026 → due 6 December 2026 |
| First day of last period (longer cycle) | Adds 280 days, plus the days your cycle runs over 28 | 35-day cycle: adds 287 days |
| Conception or ovulation date | Adds 266 days (38 weeks) | Conceived 15 March 2026 → due 6 December 2026 |
| IVF, day-5 embryo transfer | Adds 261 days to the transfer date | Transfer 4 May 2026 → due 20 January 2027 |
| IVF, day-3 embryo transfer | Adds 263 days to the transfer date | Transfer 4 May 2026 → due 22 January 2027 |
Naegele's rule, the shortcut doctors use
Before apps, providers used Naegele's rule. Take the first day of your last period, add 7 days, then count back 3 months and add a year. Mayo Clinic describes the same method. Because months have different lengths, Naegele's rule and the 280-day method can land a day or two apart. Both are estimates, so a small gap doesn't matter.
Why the IVF option is different
With IVF, the date of fertilisation and the age of the embryo are known exactly. ACOG recommends that the due date for an IVF pregnancy be set from the embryo's age and the transfer date, rather than from a period date. Our IVF due date guide walks through day-3, day-5 and frozen transfers with examples.
How accurate is a pregnancy calculator?
A calculator is a good starting point, but your official due date usually comes from a scan. The American College of Obstetricians and Gynecologists (ACOG) says an ultrasound in the first trimester, up to 13 weeks and 6 days, is the most accurate way to set or confirm gestational age. At that stage the measurement is accurate to within about 5 to 7 days.
Your period-based date is most likely to be off if:
- your cycles are longer, shorter or less regular than 28 days
- you are not sure of the date your last period started
- you got pregnant soon after stopping hormonal birth control, or while breastfeeding
- you had bleeding in early pregnancy that you mistook for a period
According to Mayo Clinic, if your period-based date and an early ultrasound date differ by more than seven days, the ultrasound date is usually used. ACOG adds that once the due date is set, it should only change in rare cases. So if your midwife or doctor gives you a date, use theirs in your plans and in your app.
What your due date really means
Your due date is the day you reach 40 weeks, not a prediction of the day labor starts. ACOG notes that most people give birth between 38 and 41 weeks. The NHS describes a normal pregnancy as lasting 37 to 42 weeks from your last period. A few useful milestones to plan around:
| Milestone | When |
|---|---|
| End of the first trimester | 13 weeks 6 days |
| Dating scan (typical window) | 11 to 14 weeks |
| Anatomy scan (typical window) | 18 to 22 weeks |
| Third trimester begins | 28 weeks |
| Full term begins | 39 weeks (37–38 weeks is "early term") |
| Due date | 40 weeks |
For a full list of visits and tests by week, see our pregnancy appointment schedule.
What to do after you get your due date
- Check how far along you are. Use How Many Weeks Pregnant Am I? to see your exact week and day today.
- Book your first prenatal appointment. ACOG suggests starting prenatal care in the first trimester, ideally before 10 weeks after your last period. In the UK, you can refer yourself to NHS maternity services as soon as you know.
- Read about this week. Our pregnancy week by week guide shows your baby's size and common symptoms for each week.
- Update the date if it changes. If your dating scan moves your due date, enter the new date wherever you track your pregnancy.
Other ways to calculate your due date
- From a conception or ovulation date: if you tracked ovulation or know the exact date you conceived, our guide to the due date calculator from conception explains the 266-day method and its limits.
- From IVF transfer: see the IVF due date guide.
- From an ultrasound: your sonographer measures the baby (crown-rump length early on) and the scan report gives your gestational age and due date. Use that date.
Save your due date in Momi
The calculator on this page works in your browser and doesn't store your dates. If you want to keep your due date and follow your pregnancy from here, the Momi app estimates your due date from your last period or conception date. It then shows your progress week by week, with your baby's size and development, a pregnancy calendar, and reminders for appointments and medicine.
Download Momi free on the App Store or Google Play. The app is free with ads, and an optional Premium upgrade is available.
Sources
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
- NHS, Due date calculator
- Mayo Clinic, Prenatal care: first trimester visits
- ACOG, How to Tell When Labor Begins
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
How does a pregnancy due date calculator work?
It adds 280 days (40 weeks) to the first day of your last menstrual period. If you enter a conception date it adds 266 days, and for IVF it counts from the transfer date and the embryo's age. The result is an estimate that your doctor or midwife may adjust after an early ultrasound.
Can pregnancy calculators be wrong?
Yes, they can be off, mostly because they assume a 28-day cycle with ovulation on day 14. Long or irregular cycles, uncertain period dates and recent hormonal birth control all reduce accuracy. An ultrasound in the first trimester is the most accurate way to date a pregnancy.
How far along am I if I know my due date?
Count back 280 days from your due date to find your pregnancy start date, then count the weeks and days from that date to today. The How Many Weeks Pregnant Am I? calculator on this site does this for you.
What is EDD and LMP in a pregnancy calculator?
EDD means estimated due date, the day you will be 40 weeks pregnant. LMP means last menstrual period, and calculators use its first day, not the last day of bleeding, as the starting point of pregnancy.
Will my baby actually arrive on my due date?
Probably not on that exact day. ACOG notes that most babies are born between 38 and 41 weeks. Think of your due date as the middle of a window, and plan for the weeks around it.
Can I use this calculator for an IVF pregnancy?
Yes. Choose the IVF option and enter your transfer date and whether it was a day-3 or day-5 embryo. Use the date your fertility clinic gives you if it differs, because it is based on your exact transfer details.