Pregnancy due date calculator
By Solesia, from public clinical guidanceUpdated
Your due date is the day you are counting towards, and it is worked out from something that happened before you were pregnant: the first day of your last period. Enter that date below and you will get the date your baby is due, how many weeks along you are today, and the page for the week you are in.
It is an estimate, and everyone involved treats it as one. About one baby in twenty arrives on the due date itself. The rest come in the weeks around it, which is why the whole of 39 and 40 weeks is called full term.
When was the first day of your last period?
If you are not sure of the exact day, the closest one you remember is enough to start with.
Leave it at 28 if you do not know. A longer or shorter cycle moves the due date by the same number of days.
How the date is worked out
A pregnancy is counted as 280 days — forty weeks — from the first day of your last menstrual period, the date your provider may write down as the LMP. That is the rule they use too, and it has an odd consequence worth knowing: you are counted as two weeks pregnant before conception happens, and about four weeks pregnant by the time a test turns positive.
The calculation assumes a 28-day cycle, with ovulation around day 14. If your cycle is longer or shorter, ovulation moves with it, so the calculator shifts the due date by the same number of days. A 32-day cycle puts the due date four days later than the standard rule would.
How accurate it is
As accurate as the date you entered. If you are sure of the first day of your last period and your cycles are regular, it is a good estimate. If you are not sure, or your cycles vary, it is a rough one.
An ultrasound in the first trimester measures the baby and dates the pregnancy from that, which does not depend on remembering a date or on the cycle being typical. Where the two disagree by more than a few days, the scan is the one that is kept, and the due date is moved.
What the weeks are called
Weeks 37 and 38 are early term. Weeks 39 and 40 are full term. Week 41 is late term, and anything from 42 weeks is post-term. These are not labels for how well things are going — they are the words your provider will use, and they decide when a conversation about induction starts.
The weeks people look up first
- 8 weeks pregnantThe first prenatal visit usually falls somewhere around here.
- 12 weeks pregnantThe dating scan, which is what your due date may be corrected by.
- 20 weeks pregnantHalfway, and the long anatomy scan.
- 40 weeks pregnantThe week your due date sits in.
Every week from 4 to 40 has a page of its own: pregnancy week by week.
Common questions
How many weeks pregnant am I?
How far along you are is counted from the first day of your last period, not from conception. Enter that date in the calculator and it gives you the week and the day you are on today, along with the page for that week.
Can I work out the due date from the conception date?
Yes, roughly: add 266 days to the day you conceived. Most people know the date of their last period more precisely than the date of conception, which is why the calculation is normally done that way round.
What if my cycle is not 28 days?
Enter your own cycle length. The due date moves by the difference: a 31-day cycle puts it three days later, a 25-day cycle three days earlier. If your cycles vary a lot, treat the result as approximate and let the first scan settle it.
Is 40 weeks the same as nine months?
Not quite. Forty weeks is a little over nine calendar months, because months are longer than four weeks. Pregnancy is counted in weeks for exactly this reason, and every week has its own page here.
Can the due date change?
It can, and often does. The ultrasound in the first trimester is more accurate than a remembered date, so if the two disagree by more than a few days your provider will move the due date to the one the scan gives.
Written from public clinical guidance, including ACOG: Methods for Estimating the Due Date, ACOG: Definition of Term Pregnancy and MedlinePlus: Pregnancy. This page is general information, not medical advice, and does not replace your provider.