Find your conception date — or your baby's — in a few clicks, plus a few fun facts about that moment in time.
Enter your birthdate to see your estimated conception week.
Choose whichever date you have on hand — all three work.
Nobody can pinpoint the exact hour a pregnancy began — sperm can survive inside the body for several days before ovulation, and ovulation itself doesn't happen on a fixed schedule for everyone. Because of that, every method below produces an estimated window rather than a single guaranteed date. What changes between methods is which piece of information you're starting from, and how precise that starting point is.
This is the method most people already have the information for. Ovulation typically happens somewhere between 11 and 21 days after the first day of your last period, depending on your cycle length — a shorter cycle shifts that window earlier, a longer one shifts it later. It's the least precise of the pregnancy-based methods, but it's also the easiest to use without needing any medical paperwork on hand.
A due date is usually set by counting forward 40 weeks (280 days) from the first day of your last period, which means it can be worked backward just as easily: subtract roughly 266 days from the due date to land on an estimated conception date. This method is convenient if your provider gave you a due date but you don't remember your last period's exact start date.
Early ultrasounds measure the size of the embryo or fetus to estimate gestational age at the time of the scan, and that measurement tends to be the most reliable of the three pregnancy-based methods — especially when done in the first trimester, before growth rates start to vary more between individual babies. If you have an ultrasound report with a gestational age on it, this method will usually give you the tightest estimated range.
If you're not currently pregnant and just curious about your own conception, the math runs the same way as the due-date method, just using your birthday instead: a full-term pregnancy is about 266 days from conception to birth, so counting back that far from any birthdate gives a reasonable estimated week. It won't be exact — a baby born a little early or a little late shifts the real number — but it lands in the right neighborhood for the vast majority of birthdates.
The words get used interchangeably in everyday conversation, but medically they describe two different moments. Fertilization is the instant sperm and egg combine, which happens in the fallopian tube. Conception, in the strict medical sense, isn't considered complete until the fertilized egg has also implanted in the uterine wall — a process that typically takes several days after fertilization. In practice, calculators like this one treat "conception" as shorthand for that whole early window, since the few days' difference between fertilization and implantation doesn't meaningfully change the estimated date range you're looking for.
All of these methods produce an estimated range, not a confirmed date — even a clinical ultrasound has some margin of error. Think of the result as "most likely conceived somewhere in this window" rather than a certainty. If you need a medically confirmed date for legal or clinical reasons, that's a conversation for your healthcare provider, not a calculator.
Close, but not quite. "Nine months" is a rough shorthand — the actual standard used by calculators (and most providers) is 266 days from conception to due date, which is slightly different from nine calendar months depending on which months are involved. That's why this tool works in days rather than months under the hood.
The last-period method assumes a fairly typical cycle, so irregular cycles widen the realistic margin of error beyond what's shown. If your cycle varies a lot month to month, a due-date or ultrasound-based estimate will generally get you closer to the real answer.
Yes — the same 266-day math still applies, it's just less precise without a specific day to count back from. Counting back roughly nine months from anywhere in a given birth month will land you in the general conception period, even if you don't have the exact date narrowed down.
Early ultrasound measurements are physically measuring the embryo or fetus itself, rather than working backward from a remembered date, which removes a lot of the guesswork involved in the other methods. That's why it's generally treated as the most reliable estimate among the three pregnancy-based options.