Due date and gestational age, with the assumptions shown

From the first day of the last menstrual period. This is the single most misunderstood fact in the subject. The forty-week clock starts about two weeks before conception, which means that at "six weeks pregnant" conception was roughly four weeks ago, and that weeks one and two of a pregnancy are — by this convention — before there is a pregnancy. The convention exists because the date of the last period is something people usually know, and the date of conception usually is not. Every clinician, every chart, and every scan report uses it.

Naegele's rule: 280 days from the first day of the last menstrual period. The old mnemonic is the same arithmetic — add a year, subtract three months, add seven days. From a known conception date it is 266 days instead, and the fourteen-day difference between the two is exactly the assumed gap between the period and ovulation.

Yes, and most calculators ignore it. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles run 35 days, ovulation is around day 21, so conception happened a week later than the rule assumes and the estimate should move a week later too. A 24-day cycle moves it four days earlier. For genuinely irregular cycles the period-based estimate is not reliable at any cycle length, and dating is normally established by an early ultrasound instead.

When the two disagree by more than a published threshold, and the threshold depends on how far along the scan was. ACOG Committee Opinion 700 sets it at more than 5 days up to 8w6d, more than 7 days from 9w0d to 15w6d, more than 10 days from 16w0d to 21w6d, more than 14 days from 22w0d to 27w6d, and more than 21 days from 28w0d. The thresholds widen because ultrasound dating loses precision as pregnancy advances — a measurement late in pregnancy tells you more about how big the baby is than about how old it is. Inside the threshold, the period-based date stands and the scan is treated as confirming it.

Because IVF dating does not go through the period at all. For a day-3 transfer the estimate is the transfer date plus 263 days; for a day-5 blastocyst, plus 261. The embryo was already three or five days old when it was transferred, which is why the offsets differ. IVF dating is the most precise dating available — the date of fertilisation is known exactly rather than inferred — and ACOG guidance is that it should not be overridden by a later ultrasound, however far the scan measures off.

No, and this changed in 2013. ACOG and the Society for Maternal-Fetal Medicine replaced the single "term" label with four: early term is 37w0d to 38w6d, full term is 39w0d to 40w6d, late term is 41w0d to 41w6d, and post-term is 42w0d and beyond. The change was made because outcomes at 37 and 38 weeks are measurably different from outcomes at 39 and 40, and calling them all "term" hid that.

Unlikely. Only a small minority of births happen on the estimated date, and a due date is better understood as the midpoint of a window than as an appointment. That is why this tool reports the term boundaries alongside the single date — the useful question is usually which window you are in, not which day the arithmetic produced.

You can estimate it: 266 days before the estimated due date, or about two weeks after the first day of the last period. It is an estimate and not a fact. Sperm can survive several days in the reproductive tract, so intercourse and fertilisation are not the same date, and ovulation does not always land where the cycle length predicts. This tool reports the estimate and labels it as one. It is not evidence of anything.

The dating arithmetic is the same — gestational age is gestational age — but almost everything downstream of it is different. Multiple pregnancies are managed on a different timeline, and the term classifications here are not the right frame for one. This tool does not know whether a pregnancy is a multiple, and nothing it outputs should be read as guidance for one.

No. This is arithmetic on the dates you type in. It cannot see an ultrasound, a chart, or a person; it does not know about multiples, complications, or anything in your history. Clinical dating is done by a clinician looking at the whole picture, and the ACOG thresholds this applies are a rule of thumb within that picture rather than a replacement for it. The disclaimer is returned as a field in the JSON and MCP responses too, so an agent relaying the numbers gets it as well.