Free health tool
Enter the first day of your last period to see your estimated due date, how far along you are today, and when each trimester starts.
Your due date
The day bleeding started, not the day it stopped.
Days from the start of one period to the start of the next. Leave it at 28 if you are not sure.
This is an estimate from the dates you enter, not a diagnosis. A dating scan measures the baby directly and replaces it.
01
Pregnancy is dated from the first day of your last period, not from conception, and it runs for 40 weeks from there. Before 37 weeks is preterm, 42 weeks and over is post-term, and the weeks in between are when almost every baby actually arrives.
The table below is the whole 40 weeks, split the way maternity services split them.
1 – 13
14 – 27
28 – 36
37 – 38
39 – 40
41
42 and over
02
The estimate is Naegele's rule: 280 days, or 40 weeks, from the first day of your last period. That assumes a 28-day cycle with ovulation around day 14.
If your cycles are longer or shorter, ovulation moves with them, so the calculator shifts the date by the difference. A 32-day cycle puts the estimate four days later; a 24-day cycle brings it four days earlier.
Worked example: a last period starting on 1 January with a 28-day cycle gives 8 October. The same period with a 32-day cycle gives 12 October.
03
A calculator can only work with the dates you give it. A scan measures the baby, which is why its date is the one your maternity team keeps using afterwards.
04
An estimated date is where antenatal care starts, not the whole of it. These are the consultations that take it further.
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The arithmetic is exact; the assumptions behind it are not. Only about one pregnancy in twenty ends on its estimated date, and the great majority of births happen in the weeks either side of it. A first-trimester scan is the accurate version, and once you have had one, its date is the one that is used.
Pregnancy is counted from the first day of your last period, not from conception — which is why you are already around four weeks pregnant when a test first turns positive. Conception happens roughly two weeks after that first day, so the pregnancy is always dated about two weeks 'older' than the embryo.
Nine completed weeks and three days since the first day of your last period. Maternity records are written this way because a whole week is too coarse when a scan or a test has to happen inside a particular window.
You can, and entering your usual cycle length will get you closer than the 28-day default. But an irregular cycle is exactly the case where the last period is a weak anchor, so treat the result as a rough guide until you have been scanned.
Often, yes — usually by a few days. The scan measures the baby instead of counting from a remembered date, so it is the one that is kept. A change of a week or more is common when the date of the last period was uncertain.
No. The calculation runs entirely in your browser. Nothing you type is sent to our servers or saved.
Either through your family doctor or directly with an obstetrician-gynaecologist: the pregnancy is confirmed, you are taken onto their list and the first tests are ordered. The scan is arranged there, and the date it gives is the one used in your pregnancy records afterwards.
The first-trimester scan is usually done between about 11 and 14 weeks, with the fetal anomaly scan around week 20. Until then the estimate above is what you have — and our registered doctors can discuss early symptoms or medicines you already take by video.
Talk to a registered doctor by video about symptoms, the medicines you already take, or anything you would rather not sit on until your first appointment.
