Roman Numeral Calculator
Convert to and from Roman numerals and do arithmetic in them.
Conception Calculator
When conception most likely happened, and the window around it.
Gestational age today
The conception window
Sperm survive up to five days and the egg about a day, so any of these days could have produced a pregnancy dated the same way.
| Date | Relative to ovulation | Chance of conception |
|---|
Why gestational age is two weeks ahead
Pregnancy is conventionally dated from the first day of your last menstrual period, which is roughly two weeks before ovulation. That means at eight weeks pregnant the embryo is about six weeks old. The convention exists because the date of a period is something you can actually observe, whereas the moment of conception almost never is.
Most likely conception date
—
Related dates
Conception window—
Last menstrual period—
Estimated due date—
Implantation (approx.)—
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Embryonic age today, two weeks behind gestational age
A conception date derived from a due date is an estimate built on another estimate. Where the exact date matters, an early dating scan is far better evidence than any calculation. This tool is informational and not a substitute for medical or legal advice.
Quick Tool Overview
The Conception Calculator works backwards from a due date, last period or current gestational age to the date conception most likely occurred. It reports a window as well as a single date, because sperm survival makes a window the honest answer.
- What it does
- Estimates the conception date and the window around it
- You give it
- A due date, last period date or current gestational age
- You get back
- A most-likely date plus the six-day window it sits in
- Best for
- Understanding pregnancy dating and the two-week offset
About the Conception Calculator
Work backwards to the date conception most likely occurred, from a due date, the first day of your last period, or how many weeks pregnant you are now. A window is given alongside the single most likely date, because a window is the honest answer.
The reason is biological rather than mathematical. Sperm survive in the reproductive tract for up to five days, while an egg is viable for roughly twelve to twenty-four hours after ovulation, so intercourse on any of about six days can result in a pregnancy dated to the same day. Pregnancy is also conventionally dated from the last period rather than from conception, which means gestational age is about two weeks greater than the embryo's actual age — a discrepancy this calculator makes explicit rather than hiding.
Key features of the Conception Calculator
Three starting points
Due date, last menstrual period or current gestational age — whichever you actually have.
A window, not false precision
Sperm survive up to five days and the egg about a day, so roughly six days can produce the same dated pregnancy.
Cycle length adjustment
Ovulation timing shifts with cycle length, which moves the conception estimate along with it.
The two-week offset explained
Gestational age is counted from your last period, so it runs about two weeks ahead of the embryo actual age.
Private by design
Dates are calculated in your browser and never transmitted anywhere.
How to use the Conception Calculator
-
1
Choose what you know
Your due date, the first day of your last period, or your current gestational age.
-
2
Adjust your cycle length
Ovulation timing shifts with cycle length, and so does the conception estimate.
-
3
Read the likely date
The single most probable date, plus the fertile window it sits inside.
-
4
Note the two-week offset
Gestational age is counted from your last period, so it runs about two weeks ahead of embryonic age.
Frequently asked questions
Subtract 266 days from your due date, or add roughly 14 days to the first day of your last period if you have a 28-day cycle. Both give an estimate rather than a certainty.
About six days. Sperm survive up to five days and the egg is viable for around a day after ovulation, so intercourse across that span can produce the same dated pregnancy.
Because pregnancy is dated from the first day of your last period, which is roughly two weeks before ovulation. At 8 weeks pregnant, the embryo is about 6 weeks old.
No. It narrows the likely window, but a due date is itself an estimate. Where the exact date matters legally or medically, an early dating scan is the better evidence.
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