Most due date calculators do one thing: last period plus 280 days. That is a fine starting point, but it is not what your own doctor actually uses once more information exists - an early scan, a known conception date, an IVF transfer date, or a cycle that is not exactly 28 days. This uses whichever of those you actually have, the same way a clinician would, and shows the honest range around the date rather than a single number that pretends to be more precise than it is.
The standard formula - known as Naegele's Rule, in use since 1812 - adds 280 days (40 weeks) to the first day of your last period. That assumes a 28-day cycle with ovulation on day 14, which is an average, not everyone's actual cycle. This adjusts for that: the correction is simply (your cycle length minus 28) days added to the standard estimate, so a 35-day cycle shifts the date 7 days later and a 21-day cycle shifts it 7 days earlier - because a longer cycle means ovulation happens later, and a shorter cycle means it happens earlier, relative to the last period.
An early ultrasound measures the baby directly and states a gestational age - "12 weeks 3 days," for instance - as of the scan date. This works backwards from that: due date = scan date + (280 days minus the gestational age in days). ACOG guidance sets the threshold for favoring the ultrasound date on a sliding scale, not one flat number: as little as 5 days at an early scan (up to 8 weeks 6 days), widening to 7 days through the rest of the first trimester, 10 days in the mid-second trimester, and further out for later scans, since a baby's size naturally varies more as pregnancy progresses. Enter both dates and this applies the correct threshold for exactly how far along the scan was, not an approximation.
If you know the actual date of conception - not estimated from your period, but actually known - the standard estimate is 266 days (38 weeks) from that date, which is 2 weeks less than the 280-day last-period figure, since conception itself typically happens about 2 weeks after the last period began.
This is the most precise method available, since the embryo's exact age is known at the moment of transfer. Due date = transfer date + (266 days minus the embryo's age in days at transfer). A day-5 blastocyst transfer means 261 days remain; a day-3 transfer means 263 days remain.
This catches people off guard more than almost anything else in early pregnancy, and it is rarely explained well. A last-period-based estimate assumes a textbook 28-day cycle and ovulation right on day 14 - reasonable as a starting guess, but it is still a guess. An ultrasound measures the baby itself, which is a direct observation, not an assumption. When the two disagree by more than about a week in the first trimester, it almost always means the original assumption was slightly off, not that anything is wrong with the pregnancy. Once a due date is set from a good first-trimester scan, it is not usually revised again later, because later scans measure size, and babies naturally start to vary more in size than in age as pregnancy progresses.
The 40-week estimate above is calibrated for a single baby. Twin pregnancies run shorter on average - guidance for uncomplicated twin pregnancies generally points toward delivery around 38 weeks rather than 40, and it depends further on whether the twins share one placenta or two: pregnancies sharing a single placenta are typically delivered earlier than those with separate placentas, because of higher monitoring needs. This is general, widely published information, not a specific recommendation for your pregnancy - your own care team will set the actual plan based on your situation.
It estimates. Only a small fraction of babies arrive on their exact estimated date - most arrive within about two weeks either side of it, which is completely normal and expected, not a sign anything is off. Treat the date as the center of a real range, not an appointment. This is general information, not medical advice - your own care provider's estimate, especially once confirmed by an early scan, is the one to actually plan around.
The standard method, called Naegele's Rule, adds 280 days (40 weeks) to the first day of your last menstrual period. If an ultrasound, a known conception date, or an IVF transfer date is available, those give a more precise estimate, since they do not rely on assuming a textbook 28-day cycle.
A last-period estimate assumes an average cycle; an ultrasound measures the baby directly. When they disagree by more than the ACOG threshold for that stage of pregnancy, guidance favors the ultrasound date, since it is a direct measurement rather than an assumption.
Only around 5% of babies are born on their exact estimated due date. Most arrive within about two weeks either side of it, which is entirely normal - the date is the center of a range, not a deadline.
Yes. Twin pregnancies typically run shorter than single-baby pregnancies, commonly landing around 38 weeks for uncomplicated cases rather than 40, and depending further on whether the twins share one placenta or two.
Use the most direct measurement available. IVF transfer date is the most precise, since the embryo's exact age is known. A first-trimester ultrasound is next most reliable, followed by a known conception date, with last-period dating as the fallback when nothing more precise is available.