Conception date calculator

How conception date is estimated

This calculator offers two ways to estimate a conception date, depending on which reference date you already know. If you know your due date, the calculator works backward using the standard 266-day rule: a full-term pregnancy is typically 38 weeks (266 days) from the moment of conception itself, as opposed to the more commonly cited 40 weeks (280 days), which is instead counted from the first day of your last menstrual period. Subtracting 266 days from a due date gives an estimated conception date that lines up with this 38-week convention.

If instead you know the first day of your last period, the calculator uses the same ovulation-timing logic as our ovulation calculator: your average cycle length minus 14 days, counted forward from the first day of your last period. This works because the luteal phase, the days between ovulation and the next period, is fairly consistent at around 14 days regardless of overall cycle length, while the days before ovulation are what actually vary between shorter and longer cycles.

These two methods give slightly different reference points because they are anchored to different biological events. The due date method counts back from conception itself, a fixed 266-day span. The LMP method instead estimates forward from the start of your last period to the day ovulation most likely occurred. Both are aiming at the same underlying event, so a small gap of a few days between the two estimates is completely normal and expected rather than a sign either method is wrong.

Why this is an estimate, not an exact date

Even with a completely regular cycle, ovulation timing shifts somewhat from month to month due to stress, illness, travel, sleep changes and other everyday factors. A cycle that averages 28 days might see ovulation happen a day or two earlier or later in any given month, which shifts the true conception date within a similar range around the number this calculator produces.

Sperm can also survive inside the reproductive tract for up to about five days under favorable conditions, meaning conception could technically result from intercourse that happened several days before the egg was actually released. Because of this, clinicians and researchers generally treat conception date as a window of several days surrounding the estimate rather than a single fixed day, and this calculator's result should be read the same way.

What conception date estimates are commonly used for

Most people use a conception date estimate for rough timeline planning, such as figuring out roughly when a pregnancy likely began relative to a trip, an event or a medical appointment, or simply out of curiosity about the pregnancy's early timeline. It can also be a helpful starting point for conversations with a healthcare provider about early pregnancy dating.

This tool is not suitable for legal or paternity purposes. Because the underlying estimate can be off by several days in either direction, it does not meet the precision required for legal determinations, custody matters or paternity questions. Situations that require that level of accuracy rely on clinical methods such as early dating ultrasound or genetic testing, not a date-based estimate like this one.

Frequently asked questions

How is conception date estimated?

If you know your due date, conception is estimated as 266 days before it, since full-term pregnancy is typically 38 weeks from conception. If you know the first day of your last period, conception is estimated using ovulation timing: cycle length minus 14 days, counted from that date.

Why do the due date method and the LMP method give different reference points?

The due date method counts backward 266 days from conception itself, a 38-week span. The LMP method instead estimates when ovulation happened after your last period, which is roughly two weeks after the period starts. Both are trying to reach the same real event, just working from different known dates, so small differences in the estimate are normal.

How accurate is a conception date estimate?

It is an estimate window of a few days, not a precise date. Ovulation timing varies from cycle to cycle even for people with regular periods, and sperm can survive inside the body for several days before an egg is released, which widens the window during which conception could realistically have occurred.

Can I use this calculator for legal or paternity purposes?

No. This tool is intended for rough timeline planning and curiosity only. It is not precise enough for legal, paternity or other determinations that require exact dating. Those situations require clinical dating methods such as early ultrasound or genetic testing.

What if I don't know my exact last period date or due date?

Use your best estimate for either field. If neither date is well known, an early dating ultrasound from a healthcare provider will give a more reliable starting point than either method used here.

This calculator provides estimates for information only, not medical or legal advice. Conception date estimates are not precise enough for legal or paternity determinations. Consult a healthcare provider for accurate pregnancy dating, typically via early ultrasound.