Due Date Calculator From Conception or Ovulation
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: To use a due date calculator from conception, add 266 days (38 weeks) to the date you conceived or ovulated. If you conceived on 10 June 2026, your estimated due date is 3 March 2027. Providers still count pregnancy from your last period, so your "weeks pregnant" will be about two weeks more than the time since conception.
Use our due date calculator and choose "conception date", or follow the steps below to work it out yourself.
How to calculate your due date from conception
Due date = conception date + 266 days
That's 38 weeks, the average time from fertilisation to birth. It lines up with the standard method of adding 280 days (40 weeks) to the first day of your last period, because in a textbook 28-day cycle you ovulate about 14 days after your period starts.
| Conception or ovulation date | + 266 days = estimated due date |
|---|---|
| 15 March 2026 | 6 December 2026 |
| 10 June 2026 | 3 March 2027 |
| 3 September 2026 | 27 May 2027 |
Step by step
- Find your conception date. If you tracked ovulation, use your ovulation day.
- Add 38 weeks (266 days). A calculator is easiest, because months have different lengths.
- To get your gestational age today, count the weeks since conception and add two.
- Check the result against your dating scan when you have it.
Due date calculator based on ovulation
Most calculators treat your ovulation day as your conception day. That's reasonable: an egg survives for about 24 hours after it's released, so fertilisation happens close to ovulation.
The catch is sex and conception aren't always the same day. Cleveland Clinic notes that sperm can live inside the body for up to five days. If you had sex on 1 September and ovulated on 4 September, conception probably happened on the 4th. Using the 1st would give a due date three days early.
| If you know... | Use this date | Accuracy |
|---|---|---|
| Ovulation confirmed by tests, temperature or a scan | Ovulation day | Good |
| Only one day you had sex that cycle | That day (or up to a few days after) | Fair: could be a few days out |
| IVF transfer date and embryo age | Transfer date, see the IVF due date guide | Very good |
| Neither | First day of your last period | Fine for regular cycles |
If your cycles aren't 28 days
This is where a conception or ovulation date really helps. Ovulation usually happens about 14 days before your next period, not 14 days after your last one. So:
- in a 35-day cycle, you'd ovulate around day 21, and a period-based due date would be about a week too early
- in a 24-day cycle, you'd ovulate around day 10, and a period-based date would be a few days too late
If you track ovulation with a fertility app or tests, a pregnancy tracker based on ovulation date can be more accurate for you than one based on your period. The NHS calculator also lets you adjust for your cycle length.
Why is pregnancy not calculated from conception?
It surprises a lot of people that, on the day of a positive test, they're already "4 weeks pregnant" even though they conceived two weeks ago. There are practical reasons providers count from the last period:
- Most people know their period date. Few know exactly when they conceived.
- Conception is invisible. There's no symptom that marks it, and sperm survival means it may happen days after sex.
- It keeps everyone on one system. Scans, growth charts, screening windows and milestones such as "full term at 39 weeks" are all based on gestational age from the last period.
So there are two ages in play. Gestational age counts from your last period, which is what your provider, scans and apps use. Fetal (conceptional) age counts from conception and is about two weeks less. When this site or Momi says "week 12", it means gestational age.
What your provider will do
Even if you know your conception date, your provider will likely confirm your due date with an ultrasound. ACOG says a scan in the first trimester (up to 13 weeks 6 days) is the most accurate way to establish gestational age, to within about 5 to 7 days. Mayo Clinic explains that if the period-based date and an early scan differ by more than seven days, the scan date is usually used.
For pregnancies from assisted reproduction such as IVF, ACOG recommends using the embryo's age and transfer date, because the conception date is known exactly.
Tell your provider your conception or ovulation date if you know it, especially if your cycles are irregular. It helps them interpret the scan.
Worked example
Amira has 33-day cycles and used ovulation tests. Her last period started on 1 June 2026, and she had a positive ovulation test on 18 June, ovulating around 19 June.
- From her period (+280 days): 8 March 2027
- From ovulation (+266 days): 12 March 2027
The ovulation date gives a due date four days later, which better reflects her longer cycle. Her 12-week scan will set the final date.
Save your conception date in Momi
Momi estimates your due date from either your last period or a conception date. Once it's set, the app follows your pregnancy week by week, with your baby's size and development, and keeps your diary, bump photos and appointments together. If your scan changes the date, just update it.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Related: due date calculator · how many weeks pregnant am I? · IVF due date · pregnancy week by week
Sources
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
- Cleveland Clinic, Due Date Calculator
- NHS, Due date calculator
- Mayo Clinic, Prenatal care: first trimester visits
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 do I calculate my due date from conception?
Add 266 days, or 38 weeks, to your conception date. For example, if you conceived on 10 June 2026, your estimated due date is 3 March 2027. This gives the same result as adding 280 days to a last period that started two weeks before conception.
Can I use my ovulation date as my conception date?
Usually, yes. Fertilisation happens within about a day of ovulation, so most calculators treat the ovulation date as the conception date. If you confirmed ovulation with tests or temperature tracking, it's a reasonable estimate.
Why is pregnancy not calculated from conception?
Because most people don't know the day they conceived. Sperm can survive for several days, so conception may happen days after sex. The first day of your last period is easier to know, so providers count from that date and call it gestational age.
How many weeks pregnant am I if I conceived 4 weeks ago?
About 6 weeks. Gestational age counts from your last period, which is roughly two weeks before conception, so add two weeks to the time since conception.
Is a due date from conception more accurate than one from my last period?
It can be if your cycles are irregular and you know when you ovulated. But for most natural conceptions, a first-trimester ultrasound is the most accurate method. Only IVF gives an exactly known conception date.
Can I enter my conception date in a pregnancy tracker?
Yes. Momi can estimate your due date from a conception date as well as from your last period, and our free due date calculator accepts conception and IVF transfer dates too.
Related guides
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
