Due Date Calculator
Pregnancy due date from your last period, IVF or a scan. Runs in your browser.
About this due date calculator
This due date calculator estimates when your baby is due and how far along you are today. It works from the first day of your last period, a known conception date, an IVF transfer date, an ultrasound, or a due date your clinic already gave you.
You also get your current week and trimester, the 37 to 42 week window when most babies arrive, and dates for common scans and tests. Had a scan? Compare it with your period dates to see whether the usual ACOG rule would move your due date.
Nothing is stored or sent anywhere. The dates stay in your browser, and the page link keeps your inputs so you can bookmark it.
Features
Five ways to start
Last period with your cycle length (20 to 45 days), conception date, IVF transfer with a day 3, day 5 or day 6 embryo, ultrasound weeks and days, or an existing due date.
How far along, on any day
Gestational age in weeks and days, trimester and percent complete for today, or for any date you pick, so you can see where you will be at a holiday or a work deadline.
Scan vs period check
Enter a scan's date and measurement. The tool applies the ACOG redating limits (5 days before 9 weeks, 7 days up to 15w6d, then 10, 14 and 21 days) and tells you which date clinics usually keep.
Milestone dates
Typical windows for the nuchal scan, anatomy scan, glucose test and group B strep test, plus the early, full, late and postterm dates, each with a real calendar date.
Week-by-week calendar
All 42 weeks with their start and end dates and trimester. Your current week is highlighted.
How to use the due date calculator
- 1Pick what you know: last period, conception, IVF transfer, a scan or your due date.
- 2Enter the date, plus cycle length, embryo age or scan weeks if asked.
- 3Read your due date, weeks pregnant, trimester and the milestone dates.
When to use it
A positive test
You know roughly when your last period started. Enter it to get a first due date before your booking appointment.
After an IVF transfer
Clinics date IVF pregnancies from the transfer, not a period. Pick the embryo's age and the date is exact to the day.
Planning leave and travel
Set the 'as of' date to a trip or a handover to see which week you'll be in. To count days between any two dates, use the date difference calculator.
Keeping a schedule
Copy the milestone dates, then print a calendar and mark your scans and appointments on it.
How is a due date calculated?
Most due dates come from Naegele's rule. Take the first day of your last menstrual period (LMP) and add 280 days, which is 40 weeks. ACOG states the same convention: the estimated due date is 280 days after the first day of the LMP.
Example: an LMP of 1 January 2026 gives a due date of 8 October 2026. The rule assumes a 28-day cycle with ovulation on day 14. If your cycles run 32 days, ovulation tends to come about 4 days later, so the calculator adds 4 days and you get 12 October.
The other methods count back to the same starting point. Conception is about day 14, so a conception date plus 266 days gives the due date. For IVF, ACOG gives transfer plus 261 days for a day 5 embryo and plus 263 days for a day 3 embryo. A day 6 embryo works out to 260 days.
Why do pregnancy weeks start before conception?
Gestational age is counted from the first day of your last period, about two weeks before you conceive. So at conception you are already '2 weeks pregnant'. Odd, but it gives everyone a date they can remember, and every chart and scan uses the same count.
Trimesters follow the same clock. The first runs to 13 weeks 6 days, the second from 14w0d to 27w6d, and the third from 28 weeks to birth.
When does an ultrasound change your due date?
An early scan measures the baby (the crown-rump length) and is the most accurate way to date a pregnancy. ACOG Committee Opinion 700 says to change the due date if the scan and the period date differ by more than a set number of days. The limit grows as pregnancy goes on, because babies vary more in size later.
- Up to 8 weeks 6 days: more than 5 days apart
- 9w0d to 15w6d: more than 7 days
- 16w0d to 21w6d: more than 10 days
- 22w0d to 27w6d: more than 14 days
- 28 weeks and later: more than 21 days
What do early, full and late term mean?
ACOG splits 'term' into bands. Early term is 37w0d to 38w6d, full term is 39w0d to 40w6d, late term is 41w0d to 41w6d, and postterm is 42 weeks or later. Before 37 weeks a birth is preterm. The calculator turns each band into real dates.
Why use Mavros
| Mavros | Typical upload sites | |
|---|---|---|
| Methods | LMP, conception, IVF day 3/5/6, ultrasound, known due date | LMP only, or LMP and conception |
| Cycle length | 20 to 45 days, adjusts the date | Fixed at 28 days |
| Scan vs period check | ACOG redating limits applied | Not offered |
| Dates for milestones | Scans, tests and term bands with calendar dates | Week numbers only |
| Privacy | Runs in your browser, nothing sent | Often asks for an email to see results |
Tips
- Count day 1 as the first day of real bleeding, not spotting.
- If your cycles are irregular, the period method is a rough guess. Your first scan will usually set the date.
- Use the earliest scan you had. A 9-week scan is more precise than a 20-week one.
- Bookmark the result. The link holds your method and dates, so you can check your week again later.
- Your weight before pregnancy guides how much gain is advised. Check your pre-pregnancy BMI if your midwife asks for it.
Questions
How accurate is my due date?
It is an estimate, not an appointment. Only a small share of babies are born on the exact day, and most arrive within about two weeks either side. A first-trimester scan narrows the estimate to within about a week.
How do I calculate my due date from conception?
Add 266 days (38 weeks) to the conception date. That is the same as 280 days from a period that started two weeks before conception.
How is an IVF due date calculated?
Add 261 days to the transfer date for a day 5 embryo, or 263 days for a day 3 embryo, as ACOG advises. For a day 6 embryo it is 260 days. Choose the embryo age in the IVF option.
Does a long or short cycle change the due date?
Yes. Naegele's rule assumes a 28-day cycle. For each day your cycle is longer, the due date moves one day later, and for each day shorter it moves one day earlier.
My scan gave a different date. Which one is right?
Usually the scan, if the gap is larger than the ACOG limit for that stage of pregnancy. Within the limit, clinics normally keep the period-based date. Your care team makes the final call.
What is the 'normal window' for birth?
Between 37 and 42 weeks. Babies born then are term. The calculator shows that window as dates, plus the full term band of 39w0d to 40w6d.
Can I use this if I don't know my last period?
Yes, if you have a scan. Choose Ultrasound and enter the scan date and the weeks and days on the report. Without a scan or a period date, a clinician will date the pregnancy for you.
Is this medical advice?
No. It gives standard estimates and typical timings. Follow the dates and tests your midwife or doctor gives you, and contact them with any concern.