How the number is worked out
Pregnancy is counted from the first day of your last period, not from conception — which is why you are already considered two weeks pregnant at the moment of conception. It is a convention rather than a biological fact, and it exists because a period is a date most people can name and a conception usually is not.
From there, the estimate is your last period plus 280 days: forty weeks. That is Naegele’s rule, and it assumes a 28-day cycle. If yours is regularly longer or shorter, ovulation happens later or earlier and the date moves with it — which is why the cycle-length field above is not decoration. A person with a 35-day cycle given a 28-day answer is handed a date a week too early.
- From a last period: that date, plus 280 days, adjusted by the difference between your cycle and 28 days.
- From a conception date: that date, plus 266 days.
- From an IVF transfer: a day-5 blastocyst transfer plus 261 days, or a day-3 transfer plus 263. IVF dating is the most precise of the three, because the one variable everything else has to estimate is known exactly.
What the date is actually worth
It is an estimate, and it should be held loosely. A dating ultrasound in the first trimester is more accurate than any calculation from a remembered date, and if the scan and the arithmetic disagree, the scan wins — your midwife or doctor will re-date the pregnancy and that date is the one that counts.
Very few babies are born on their due date. Most arrive in the fortnight around it. Thinking of it as a due fortnight rather than a due day is both more accurate and considerably kinder to the last few weeks.
What to do with it
Once you have a week number, read the week. All forty weeks are here, each with the baby’s real measurement, a drawing at that size, and its sources at the bottom.