How the sum works: two ways to date the same pregnancy
A pregnancy’s length gets measured two different ways depending on what you actually know. The most common, Naegele’s rule, counts forward from the first day of your last menstrual period (LMP) rather than from conception, because almost everyone can name that date exactly, while the day an egg was actually fertilised is rarely known and usually falls a week or two after it. The rule, named after the German obstetrician who published it in the early 1800s, adds 280 days, 40 weeks, to that first day.
If you happen to know when conception actually occurred, perhaps from ovulation tracking or a single-cycle attempt, the sum runs from there instead: 266 days, 38 weeks, because conception typically lands around two weeks after the LMP that Naegele’s rule counts from. Both formulas point at the same calendar date for an average 28-day cycle; they are simply two ends of the same measuring tape.
Why your cycle length changes the sum
Naegele’s rule assumes a 28-day cycle with ovulation on day 14, exactly halfway through. Real cycles run anywhere from about 21 to 35 days and still count as normal, and ovulation moves with the cycle: someone with a 35-day cycle usually ovulates around day 21, a week later than the 28-day assumption, so conception, and the resulting due date, land a week later too. This calculator applies that shift by adding the difference between your average cycle length and 28 straight onto the standard 280 days. A 32-day cycle, four days longer than average, adds four days; a 24-day cycle, four days shorter, takes four away.
That correction only helps when your cycles are fairly regular. If your cycle length varies by more than a few days from month to month, an early dating ultrasound will get you closer than any version of this arithmetic, since the rule has nothing steady to measure irregularity against.
IVF: starting from a date nobody has to estimate
IVF removes most of the guesswork Naegele’s rule exists to work around, because the exact age of the embryo at transfer is already known from the lab. The due date still runs on the same 266-day conception-to-birth interval used for conception dating; it is just measured backward from the transfer date instead of forward from an assumed ovulation day. A day-5 embryo, a blastocyst, has already spent five days developing outside the body, so only 261 days (266 minus 5) get added from the transfer date. A day-3 embryo, still at the cleavage stage, adds 263 days (266 minus 3).
Because the starting point is a known lab date rather than an assumed ovulation day, an IVF due date worked out this way is usually treated as being about as reliable as one confirmed by an early scan.
Trimesters and the milestones along the way
The three trimesters split the 40 weeks into roughly equal thirds, though the boundaries used here follow the common clinical convention rather than an exact three-way split: the first trimester runs up to 12w0d, the second from 12w0d to 27w0d, and the third from 27w0d to birth. Several dates along that timeline matter more than the trimester lines themselves. Most places offer a mid-pregnancy anatomy scan somewhere between 18w0d and 22w0d, checking the baby’s structural development in detail; the NHS books this within an 18 to 21-week window, most often landing around 20 weeks, and other health systems describe a near-identical 18 to 22-week window for the same scan.
Near the end of the timeline, obstetric bodies including ACOG split what used to be a single "term" label into four narrower bands, because outcomes differ measurably week by week even within what used to count as one group: early term runs 37w0d to 38w6d, full term 39w0d to 40w6d, late term 41w0d to 41w6d, and post-term from 42w0d onward. That is also why "full term" begins a week before the due date rather than on it, and why pregnancies reaching the post-term line are typically monitored more closely, with induction often discussed as that boundary approaches.
How accurate is any of this
Not very, in the sense most people mean by an exact date. Only around 4 to 5 in every 100 babies are actually born on their calculated due date; most arrive within about two weeks either side of it, and week 39 is the single most common week of all for a first due date to actually resolve into a birth. None of that makes the date useless, it is still the best single estimate available from the information behind it, but it was never built to predict a specific day, only the centre of a fairly wide, entirely normal window.
A dating ultrasound, usually done in the first trimester, measures the baby directly rather than relying on a remembered period date or an assumed cycle length, which is why it takes priority whenever the two disagree by more than about a week. If a sonographer or midwife gives you a due date that differs from anything calculated here, follow theirs; a scan-based estimate corrects for exactly the kind of cycle irregularity this page can only roughly adjust for.
Questions people ask
How accurate is my due date?
Treat it as the centre of a normal window rather than a forecast. Only around 4 to 5 in every 100 babies are born on their exact calculated due date, and most arrive within about two weeks either side of it. The date is still useful for planning and for spotting when a pregnancy is running notably early or late, just not for predicting a specific day.
Why did my due date change after my dating scan?
A first-trimester ultrasound measures the baby directly, which is a steadier signal than a remembered last period date or an estimated cycle length. When the scan and the period-based calculation disagree by more than about a week, the scan date generally takes over as the new estimate, since it corrects for exactly the kind of timing uncertainty this calculator can only roughly account for.
What if my cycle isn’t the usual 28 days?
Use the cycle length slider on the last-period tab. It shifts the due date by the gap between your average cycle and the 28-day assumption Naegele’s rule is built on, since a longer cycle means later ovulation and a later due date, and a shorter cycle the opposite. If your cycles vary a lot month to month, a dating scan will get you closer than any cycle-length correction can.
How is an IVF due date worked out?
From the transfer date plus 266 days minus the embryo’s age at transfer, since the embryo has already been developing for that many days outside the body. A day-5 blastocyst transfer adds 261 days; a day-3 cleavage-stage transfer adds 263 days. Because the starting point is a known lab date rather than an assumed ovulation day, this figure is usually about as reliable as a scan-confirmed estimate.
What’s the real difference between counting from my last period and from conception?
Conception typically happens around two weeks after the first day of the last period, which is why the two formulas differ by exactly that much: 280 days from the last period versus 266 days from conception. For a textbook 28-day cycle they land on the same calendar date; they are two ends of the same measuring tape, not two competing estimates.
What happens if I go past my due date?
Going past the due date itself is common and not automatically a concern; full term already runs from 39w0d to 40w6d, meaning the due date sits inside that range rather than marking its end. From 41w0d pregnancies move into what obstetric guidelines call late term, and monitoring generally increases from there, with induction often becoming part of the conversation as the 42w0d post-term threshold approaches. Your midwife or doctor is the right person to talk through timing with, not a calculator.
Can I use this if my cycles are very irregular?
You can, but treat the result cautiously. The cycle-length correction here assumes a fairly consistent cycle from month to month; if yours swings widely, an early dating ultrasound measures the pregnancy directly rather than working backward from an average, and will get you a more dependable estimate than any version of this arithmetic can.
This calculator gives an estimate from standard obstetric formulas; it is not a medical assessment and cannot replace a dating ultrasound, midwife or doctor. Always confirm your dates and any concerns about how a pregnancy is progressing with your own care team.