You've got the positive test, a calendar open on your phone, and you're scrolling back through the weekends trying to work out which one it was.
"When did I conceive?" has an honest answer: a window of a few days, usually about two weeks after your last period started. Below, you'll work it out from your last period, your due date, an early ultrasound or a birth date, with an example for each.
Conception usually happens around ovulation, about two weeks after the first day of your last period in a 28-day cycle. Because sperm can live for several days, the date you had sex and the date you conceived may not be the same.
Key Takeaways
- Count about two weeks forward from the first day of your last period in a 28-day cycle, or about 14 days back from when your next period was due (ACOG).
- From a due date, subtract 266 days (38 weeks) to estimate your conception date.
- A first-trimester ultrasound is the most accurate dating method, to within about 5 to 7 days (ACOG).
- The sex that led to pregnancy may have been up to 5 days before ovulation by ACOG's figures, or up to 7 days by the NHS's, so dates can't single out one encounter.
When Did I Conceive? The Short Answer
Your pregnancy weeks start before conception. MedlinePlus, the National Library of Medicine's health site, explains that because it's impossible to know exactly when conception happens, gestational age is counted from the first day of your last period. You aren't pregnant yet in weeks 1 and 2, which also makes working out how many months pregnant you are confusing.
ACOG's dating guidance, Committee Opinion 700, sets the due date 280 days after the first day of your last period, assuming a 28-day cycle with ovulation on day 14. In that textbook cycle, you conceived about two weeks after your period started, in week 3: conception. By week 40, MedlinePlus notes, it has been 38 weeks since conception.
The window comes from how long sperm and the egg survive. ACOG says an egg survives about 24 hours after ovulation, and sperm about 3 days, sometimes up to 5, so pregnancy can follow sex from 5 days before ovulation to 1 day after. The NHS gives a longer figure: up to 7 days in the fallopian tubes.
How to Calculate Your Conception Date
Pick the method that matches what you know. Every example date is made up.
From Your Last Period
Start from the first day of your last period and add about 14 days if your cycles run close to 28 days. The NHS says ovulation is usually about 14 days before the start of your next period, and that the timing depends on how long your cycle is.
Say your last period started on March 3, 2026, and your cycle is 28 days. Ovulation was most likely around March 17. By ACOG's figures, sex from March 12 to March 18 could have led to this pregnancy; the NHS's 7-day figure moves the start back to March 10.
For a longer or shorter cycle, count back from when your next period was due, since ACOG calls the stretch between ovulation and the next period the most consistent part of the cycle. With a 32-day cycle and the same March 3 start, your next period was due April 4, so ovulation was likely around March 21. The NHS puts ovulation 10 to 16 days before the next period in most women, which spreads that estimate from March 19 to March 25.
Committee Opinion 700 warns that period-based dating can't account for a misremembered date, an irregular cycle or variation in when you ovulate. If you're planning a future pregnancy, you can track your fertile window as it happens.
From Your Due Date
Subtract 266 days, which is 38 weeks. This mirrors how a period-based due date is set: 280 days after your last period, minus the 14 days before ovulation.
Say your due date is January 20, 2027. Count back 38 weeks and you land on April 29, 2026, your estimated conception date.
The window depends on how the due date was set. If it came from a first-trimester ultrasound, ACOG gives that measurement an accuracy of plus or minus 5 to 7 days, so allow a window from about April 22 to May 6. A period-based due date carries the uncertainties above.
From an Early Ultrasound
ACOG calls ultrasound measurement in the first trimester, up to 13 weeks and 6 days, the most accurate way to establish or confirm how far along you are. The NHS offers the dating scan at around 10 to 14 weeks, to check how many weeks pregnant you are and work out your due date.
To use your scan, take the gestational age it measured, subtract 2 weeks, and count that many days back from the scan date.
Say a scan on May 5, 2026 measures you at 8 weeks and 3 days. Take away 2 weeks and you get 6 weeks and 3 days, or 45 days, which puts conception around March 21, 2026. With ACOG's 5 to 7 days either way, the window runs from about March 14 to March 28.
ACOG adds that if a scan before 14 weeks differs from your period-based dating by more than 7 days, the due date should change to match the scan, so use the scan-based date for this math.
From a Birth Date
This covers the "when was I conceived, based on my birthday?" question. The starting point is your birth date minus 266 days, if you arrived on your due date.
Babies arrive across a window of weeks, from early term to late term, and MedlinePlus says a normal pregnancy lasts anywhere from 37 to 42 weeks. That makes this the widest window of the four methods. Say you were born on June 12, 1996:
| If you were born at | Count back | Estimated conception |
|---|---|---|
| 37 weeks | 245 days | October 11, 1995 |
| 40 weeks (the due date) | 266 days | September 20, 1995 |
| 42 weeks | 280 days | September 6, 1995 |
That's a five-week spread. If a parent or your birth records say how many weeks along the pregnancy was at your birth, subtract 2 weeks and count back: 39 weeks and 2 days means 261 days, to September 25, 1995.
Working forward instead? If you know when you ovulated or conceived, the due date calculator's Ovulation option adds 266 days to give your estimated due date.
If I'm 4, 5 or 6 Weeks Pregnant, When Did I Conceive?
Subtract 2 weeks from how far along you are. Your pregnancy weeks count from your last period, so conception falls about 2 weeks into that count, assuming ovulation around day 14.
| Weeks pregnant | Conception was about | If that week starts on June 1, 2026, conception was around |
|---|---|---|
| 4 weeks | 2 weeks ago | May 18, 2026 |
| 5 weeks | 3 weeks ago | May 11, 2026 |
| 6 weeks | 4 weeks ago | May 4, 2026 |
| 7 weeks | 5 weeks ago | April 27, 2026 |
Those dates move later if your cycles run longer than 28 days and earlier if they run shorter. Our guide to pregnancy week 4 covers what's happening around then.
How Accurate Is a Conception Date?
A conception date is as accurate as the method behind it, plus a few days for biology. ACOG's Committee Opinion 700 gives these accuracy ranges for ultrasound dating:
- First trimester, up to 13 weeks 6 days: about 5 to 7 days either way
- 14 weeks to 21 weeks 6 days: about 7 to 10 days
- 22 weeks to 27 weeks 6 days: about 10 to 14 days
- 28 weeks and later: about 21 to 30 days, the least reliable
Dating from your period alone is looser. ACOG reports that about half of women accurately recall the first day of their last period. In one study it cites, 40% of women given a first-trimester ultrasound had their due date adjusted because the scan and their period dates differed by more than 5 days.
Biology adds its own window. Fertilization follows ovulation within about a day, since the egg survives about 24 hours (ACOG), but the sex behind it could have been up to 5 days earlier (ACOG) or 7 (NHS). Even with a good first-trimester scan, your conception date sits in a span of up to two weeks.
Your due date is an estimate too (Naegele's rule), and conception math inherits every assumption in it. Two questions for your OB or midwife: which method set your due date, and did an early scan change it?
IVF: Why Your Date Is Exact-ish
In IVF, the day of fertilization is known, because it happens in a laboratory. ACOG explains that in IVF, sperm is combined with the egg in a laboratory, and embryos are placed in the uterus a few days after fertilization. Committee Opinion 700 says to set the due date from the embryo's age and the transfer date: 261 days after transfer for a day-5 embryo, or 263 days for a day-3 embryo.
Say a day-5 embryo is transferred on April 10, 2026. The estimated due date is December 27, 2026, and counting back 266 days from it reaches April 5, 2026: five days before the transfer, the embryo's age counted from fertilization in the lab.
The "-ish" belongs to the due date, which stays an estimate because babies arrive across a range of weeks. The calculator's IVF option takes your transfer date and a day-3 or day-5 embryo.
If I Got Pregnant Today, When Would I Be Due?
Add 266 days, or 38 weeks, to the day you conceive. Conceiving on November 1, 2026, for example, gives an estimated due date of July 25, 2027. To run it for today's date, enter today as the ovulation date in the calculator.
Frequently Asked Questions
How do I work out my conception date from my due date?
Subtract 266 days, or 38 weeks, from your due date. A due date of January 20, 2027 points to conception around April 29, 2026. If an early ultrasound set the due date, allow about 5 to 7 days either side.
What day did I conceive?
Most likely within a day of ovulation, which falls about 14 days before your next period was due. The sex that led to it may have been up to 5 days earlier (ACOG) or up to 7 days earlier (NHS).
How accurate is a conception date?
It depends on the method. ACOG puts first-trimester ultrasound dating within about 5 to 7 days, and dating from a last period alone is less reliable. A conception date carries the same margin, and the sex that led to it may have been several days earlier.
When was I conceived, based on my birthday?
Subtract 266 days from your birth date if you were born on your due date. A normal pregnancy lasts 37 to 42 weeks, so the real date could fall up to three weeks later or two weeks earlier than that estimate.
Can a due date tell you who the father is?
No. If two possible conception dates are days apart, dating can't show which one led to the pregnancy, because sperm survive for days and even early scans have a margin of about a week. A DNA paternity test can answer it.
If I'm 5 weeks pregnant, when did I conceive?
About 3 weeks ago, assuming a 28-day cycle with ovulation around day 14. Pregnancy weeks count from the first day of your last period, so subtract 2 weeks. A longer cycle moves the date later.
This article is for information only and does not replace medical advice. Bring questions about your dates to your OB or midwife.
How This Guide Was Made
The Babysential Team wrote this guide from the ACOG, NHS and MedlinePlus pages below, checked on October 11, 2026, with AI-assisted drafting. Every example date is made up, and each date and day count was calculated with a script.
Sources
- Methods for Estimating the Due Date, Committee Opinion No. 700. ACOG, 2017, reaffirmed 2025
- Fertility Awareness-Based Methods of Family Planning. ACOG, last reviewed 2025
- Treating Infertility. ACOG, last reviewed 2024
- Trying to get pregnant. NHS, 2026
- Fertility in the menstrual cycle. NHS, 2023
- 12-week scan. NHS, 2023
- Fetal development. MedlinePlus, U.S. National Library of Medicine, 2025
