How Pregnancy Due Dates Are Calculated
A due date is a reference point, not a prediction of the exact day you will give birth. Most due dates start with the first day of the last period and a 280-day estimate.
The short answer
A common method adds 280 days — about 40 weeks — to the first day of your last period. That is the basis of the due date calculator. If your average cycle is longer or shorter than 28 days, the calculator adjusts the estimate.
The 280-day rule
For a 28-day cycle, the estimate is:
Due date ≈ first day of last period + 280 days
For example, if the first day of your last period was January 1, the estimate is October 8. If your usual cycle is 35 days, the adjusted estimate is about a week later.
This convention is sometimes called Naegele’s rule. It assumes ovulation occurs about two weeks before the next period, so the cycle-length adjustment shifts the estimate when your cycle is not 28 days.
Why pregnancy dating starts before conception
Pregnancy is usually dated from the first day of the last period, not from the day of conception. In a typical 28-day cycle, conception happens around two weeks after the last period began. That is why the first two weeks of a 40-week pregnancy are counted before conception.
The pregnancy week calculator uses the same convention to estimate gestational age in weeks and days.
Why the date can change
Your maternity care team may revise the due date after an ultrasound, especially if the last period date is uncertain or your cycles are irregular. Early ultrasound dating is often more accurate than a calendar estimate. If you know the date from your care team, use that date for appointments and decisions.
Why most babies are not born on the exact date
A due date is a reference point, not an appointment. Most babies are born in a range around the estimated date, and a full-term pregnancy is commonly described as 37 to 42 weeks. Your care team will use the due date for monitoring and planning, but it cannot predict the exact day of birth.
Contact your care team when needed
Contact your maternity care team if you notice reduced fetal movement, bleeding, fluid leaking, severe pain, or regular painful contractions. If you think you or your baby may be in immediate danger, call your local emergency number.
What the date is used for
The estimated due date helps with scheduling appointments, monitoring pregnancy progress, and planning leave. It is not a guarantee of when labor will start, and it does not assess your baby’s health. Only your care team can do that.
If you do not know your last period date
An early ultrasound can provide a dating estimate. Talk with your care team about which date to use. Do not rely on a calculator when the date is uncertain — the difference can affect monitoring and decisions about your care.
Related guides
Questions about due dates
How accurate is an estimated due date?
It is a reference date, not a prediction of the exact birth day. Most babies are born in a range around it. An early ultrasound is often more accurate than a calendar estimate, especially if your cycles are irregular.
Can my due date change?
Yes. Your maternity care team may revise it based on an ultrasound or other information. Use the date they provide for appointments and decisions about your care.
What if I do not know my last period date?
Ask your care team about ultrasound dating. An early scan can provide an estimate when the last period date is uncertain. A calculator cannot replace that assessment.
Does the due date tell me when labor will start?
No. It is a reference point. Contact your care team if you have contractions, your waters break, you bleed, or your baby’s movements change.
General references
These organizations provide general health information. They are external sites with their own content and policies, and this site is not affiliated with them.