Quick answerAn estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. Pregnancy weeks are counted from that same date — about two weeks before conception actually happened — which is why you are "six weeks pregnant" when the embryo is four weeks old. This method assumes a regular 28-day cycle, so if your cycle is irregular the calculation may be off, and a first-trimester ultrasound is more accurate. Importantly, the date is an estimate, not an appointment: only a small share of women give birth on the exact day, and birth between weeks 37 and 42 is within the normal range.
How is a due date calculated?
The standard method used worldwide is simple: add 280 days — that is, 40 weeks — to the first day of your last menstrual period. This is the figure any pregnancy calculator gives you, and the starting point your doctor uses.
Note that the count starts from the first day of your period, not the day it ended and not the day of intercourse. This confusion is very common and leads to a week or more of error.
Why are pregnancy weeks counted from before conception?
This part confuses many women, and it makes sense once explained. Conception usually happens around mid-cycle, roughly two weeks after the first day of your period. But doctors count from the first day of the period because it is the only date a woman knows precisely — whereas the moment of fertilisation cannot be pinpointed.
The practical result: the gestational age in weeks is about two weeks ahead of the embryo's actual age. So when you are told you are eight weeks pregnant, the embryo formed about six weeks ago. This is normal, not a calculation error.
Why might your doctor adjust the date?
Calculating from the last period assumes your cycle is regular, 28 days long, and that ovulation happened on day 14. Many women do not fit these assumptions, which is why measuring the baby by ultrasound in the first trimester is the most accurate way to establish gestational age.
An irregular cycle, or one longer or shorter than 28 days, makes the last-period calculation less accurate.Not remembering the exact date of your last period is a common reason for adjusting the date.Conceiving right after stopping hormonal contraception, when ovulation may take time to settle.If the ultrasound measurement differs noticeably from the last-period calculation, the ultrasound date is usually adopted.A twin or higher-order pregnancy is followed on a different timeline.
Adjusting the date is not a sign that something is wrong. It is simply refining the estimate with better information, and it helps schedule tests and screenings at the right time.
Why don't most women give birth on the due date?
Because the date is a statistical midpoint, not a fixed appointment. Only a very small share of women give birth on the exact day, and the majority give birth within the two weeks around it. Birth between weeks 37 and 42 falls within the medically recognised range.
Treating the due date as a window rather than a day removes a lot of stress in the final weeks, and spares you the feeling that something is overdue simply because the date has passed.
When to see your doctor about timing
If you don't know the date of your last period or your cycle is irregular — to establish dating by early ultrasound.If you pass week 41, when closer monitoring and a discussion about induction begin.If any signs of labour appear before week 37.If the due date written in your records differs between visits without an explanation — you are entitled to ask which date is being used.
Frequently asked questions
Which day do I start counting pregnancy from?
From the first day bleeding started in your last period — not the last day of it, and not the day of intercourse. Add 280 days (40 weeks) to that date to get your estimated due date.
Why does the ultrasound say a different week than my calculation?
Because your calculation assumes a regular 28-day cycle with ovulation on day 14, which doesn't apply to everyone. Measuring the baby in the first trimester is more accurate, so it is usually adopted when there is a noticeable difference. This is a correction to the estimate, not a sign of a problem.
My cycle is irregular — how do I know my due date?
Work out a rough estimate from the last period you remember, then tell your doctor your cycle is irregular and ask for a first-trimester ultrasound. Measuring the baby at that stage is the most accurate way to establish gestational age when the period date isn't reliable.
I've passed my due date — should I worry?
Going a few days past the date is very common and not worrying in itself, since the date is an estimate rather than a set day. After week 41, though, closer monitoring of the baby begins along with a discussion with your doctor about induction. Keep following your baby's movements as usual and call immediately if they change.
This content is for educational purposes only and is not a substitute for professional medical advice.
Dawrat Hayat