Wondering when did I conceive? Estimate your conception date and likely conception window — starting from your due date or your last period.
The date from your due date calculator, first scan, or provider.
Conception is the moment a sperm fertilizes an egg — and it happens at, or very close to, ovulation. Each cycle, one of your ovaries releases a mature egg into the fallopian tube, where it waits to be fertilized. If sperm are present and one succeeds, a single cell called a zygote forms and begins dividing as it travels toward the uterus. That fertilization event is what we mean by conception, and it is the true biological start of a pregnancy.
From that point, a full-term pregnancy runs about 38 weeks (266 days) from conception to birth. Yet you'll almost always hear pregnancy described as "40 weeks." The reason is a long-standing medical convention: providers date pregnancy from the first day of your last menstrual period (LMP), which typically falls about two weeks before ovulation. So the same pregnancy is roughly 40 weeks measured from your last period and about 38 weeks measured from conception — the two-week difference is simply where the counting starts.
This is also why the week number you hear at appointments can feel confusing. When you're told you're "6 weeks pregnant," that's counted from your last period, meaning you actually conceived only about four weeks earlier. Understanding this offset makes the numbers from this calculator — and from your provider — line up.
The tool offers two ways to answer "when did I conceive?", because people arrive here from two different starting points. Everything runs in your browser — nothing you enter is stored or sent anywhere.
From your due date. If you already have an estimated due date, the calculator counts back 266 days (38 weeks from conception) to land on your estimated conception date. Around that, it shows a likely conception window of about five days on either side, reflecting the reality that ovulation and fertilization can't be pinned to a single day from a due date alone.
Conception date = due date − 266 days · likely window ≈ that date ± 5 days
From your last period. If you know the first day of your last period, the calculator estimates ovulation as your cycle length minus 14 days after that date — since the luteal phase (ovulation to next period) is fairly constant at about 14 days. That estimated ovulation date becomes your conception date. The likely window runs from 5 days before to 1 day after ovulation, matching how long sperm and egg can survive. From either mode, the tool also projects your estimated due date (conception + 266 days) and, if the conception date is already in the past, your current gestational age in weeks and days (counted from the LMP-equivalent date, which is conception plus 14 days).
It's tempting to want one exact date, but biology doesn't cooperate. The reason comes down to two numbers. Sperm can survive inside the reproductive tract for up to about 5 days while waiting for an egg. The egg, once released, is only viable for roughly 12 to 24 hours. Put those together and you get a fertile window of about six days: the five days leading up to ovulation, plus ovulation day itself.
Because of that survival gap, the day you had intercourse and the day conception actually happened can be several days apart. You might conceive from intercourse that took place four days before your egg was even released. That's perfectly normal — but it means any single "conception date" is really the most likely point within a range, not a guaranteed moment. When this calculator shows a window alongside the headline date, that range is the honest answer.
Ovulation timing itself also shifts from cycle to cycle. Stress, illness, travel, and sleep changes can nudge ovulation earlier or later, which moves conception with it. This is why calendar-based estimates work best as a well-reasoned approximation rather than a precise measurement.
There are several ways to arrive at a conception estimate, and they don't all carry the same accuracy. Here's how the common methods compare — with the important caveat that, medically, an early ultrasound is the most reliable way to date a pregnancy.
| Method | What it uses | Accuracy |
|---|---|---|
| LMP dating | First day of your last period plus your average cycle length | Good if cycles are regular; assumes day-14 ovulation, so off by days if you ovulate early or late |
| Due-date back-calculation | A known due date minus 266 days | Only as accurate as the due date it's based on; inherits any dating adjustments already made |
| Ultrasound dating | Measurement of the embryo/fetus at an early scan | Most accurate, especially before 14 weeks; the medical standard when it differs from LMP dating |
| Ovulation / OPK tracking | A recorded positive ovulation test or temperature rise | Very good when you have it; pinpoints ovulation to about a day, narrowing the conception window |
Most people combine methods: a calendar estimate to get oriented, refined by an early ultrasound once pregnancy is confirmed. If your scan-based due date differs from your calendar math, trust the scan — and your provider's guidance.
A conception calculator can be a lovely way to picture the start of your pregnancy, but it is important to be clear about what it can't do. The date it produces is an estimate, surrounded by a window of several days. It is not reliable for legal or paternity questions, and it should never be used to make those determinations. Because ovulation timing varies and sperm can survive for days, the true conception date can fall anywhere across a roughly six-day window — far too wide a range to identify a specific event or person.
If paternity is a genuine question, the only dependable answer comes from DNA testing, and medical dating questions belong with your healthcare provider. From a clinical standpoint, ultrasound is the most accurate way to date a pregnancy, particularly a scan performed in the first trimester. Treat this calculator as a friendly, informative estimate — helpful for understanding your timeline and satisfying curiosity, not for resolving anything that carries legal weight.
The simplest way is to work backward from your due date: conception happens about 266 days (38 weeks) before your baby's estimated arrival, so conception date ≈ due date − 266 days. If you don't have a due date yet, you can estimate from your last period instead — conception usually occurs around ovulation, roughly your cycle length minus 14 days after the first day of your last period. Both are estimates; ultrasound dating is more accurate than calendar math.
Not necessarily. Sperm can survive in the reproductive tract for up to 5 days, so intercourse several days before ovulation can still lead to pregnancy. Fertilization — the true moment of conception — happens when a sperm meets the egg at or shortly after ovulation, which may be days after intercourse. That gap is exactly why conception is best thought of as a window rather than a single calendar day.
A conception calculator gives a solid estimate, but it is still calendar math. It assumes an average 28-day cycle and 14-day luteal phase unless you tell it otherwise, and it cannot account for early or late ovulation in a given cycle. For medical dating, an early ultrasound (ideally before 14 weeks) is more accurate than any date-based calculation, which is why your provider may adjust your due date after your first scan.
No. A conception calculator produces an estimated date and a likely window of several days — it is not precise enough to establish paternity, and it should never be used for legal questions. Because ovulation timing varies and the fertile window spans roughly six days, the true conception date can fall anywhere in that range. Paternity can only be determined by DNA testing, and medical dating questions should go to your healthcare provider.
By long-standing medical convention, pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. That is because the LMP is a date most people can identify, while the exact moment of conception usually can't be pinned down. Conception typically happens about 2 weeks after the LMP, so a pregnancy that is 40 weeks by LMP dating is roughly 38 weeks from actual conception.