Four ways in, depending on what you know: the first day of your last period, a conception date, an IVF transfer, or a gestational age measured at a scan. All four are converted to the same anchor — the implied last period — so the due date, the trimesters and every milestone come off one number rather than four.
The last-period method adjusts for cycle length, which most calculators skip. Nothing you enter here is sent anywhere; the whole thing runs in your browser.
Because gestational age is counted from the first day of your last period, not from conception — that is the convention every clinic and every scan uses. Conception happens roughly two weeks into that count. A tool that counted from conception would report an age two weeks behind everything your doctor tells you.
Yes, and most calculators ignore it. Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycle runs 35 days you ovulate about a week later, so the due date moves about a week later too. That week matters when the date is being used to decide whether labour is early or late.
An IVF transfer date, because the embryo’s age is known exactly. After that, a first-trimester ultrasound — which in practice overrides a date-based estimate when the two disagree. Conception and last-period dates are the least precise, because both rest on assumptions about when ovulation happened.
Almost certainly not. Only about one birth in twenty happens on the estimated date. Anything from 37 to 42 weeks is normal, which is why the tool shows the full-term window and the post-term point rather than a single day.
No. Every calculation runs in your browser and there is no endpoint that would accept these dates. Nothing is stored, and closing the tab is all it takes to be rid of it.
shifted by however far your cycle differs from 28 days.
Each offset is 280 minus how far along the pregnancy already was on that date. A day-5 blastocyst is two days older than a day-3 embryo, so the same transfer date is two days further along.
A longer cycle ovulates later, so conception was later and the due date is later. A calculator that assumes 28 for everyone is a week out for a lot of people.
An estimate, and not a substitute for prenatal care. Only about one birth in twenty happens on the due date, and a first-trimester ultrasound is more accurate than any date-based method. Every question about your pregnancy belongs with your doctor or midwife. Nothing you type here is sent to our servers — the calculation runs entirely in your browser.
