The formula
How it works
Estimate a baby’s due date from the first day of the last period or the date of conception. The calculator also shows how many weeks along a pregnancy is today and which trimester it falls in.
FAQ
How accurate is a due date?
It is an estimate: only about 1 in 20 babies actually arrive on their due date, and a full-term birth is anything from 37 to 42 weeks. It is a useful marker for care and planning rather than a firm appointment.
Why start from the last period, not conception?
The date of the last period is usually known, while the exact day of conception often is not. Dating from the last period is the standard medical convention, and pregnancy is counted as 40 weeks from that day.
What if my cycle is irregular?
This calculator adjusts for a cycle longer or shorter than 28 days, but very irregular cycles make the last-period method less reliable since ovulation timing becomes harder to predict. An early ultrasound is usually more accurate in that situation.
Why did an ultrasound give a different due date?
Early ultrasounds measure the size of the embryo or fetus, which can be a more accurate way to date a pregnancy than the last period, especially if cycles are irregular or the last period date is uncertain. Doctors often adjust the due date if the two estimates differ by more than about a week.
Does this work for twins or multiples?
The 280-day calculation is the same starting point, but pregnancies with twins or multiples commonly deliver earlier than a single-baby pregnancy, often around 36 weeks. A doctor will usually set expectations specific to a multiple pregnancy rather than relying on the standard due date.
What happens if the due date passes with no birth?
Going past the due date is common and not unusual on its own, since full term spans 37 to 42 weeks. Doctors typically monitor the pregnancy more closely after 40 weeks and may discuss inducing labor if it extends well beyond that.
Why does the calculator show weeks and days instead of months?
Pregnancy is tracked in weeks because it gives a more precise picture of development than months do, and it lines up with how doctors schedule checkups and milestones. Counting from the last period, 40 weeks works out to roughly nine months.
About the due date calculator
This calculator estimates when a baby is due, based on either the first day of the last menstrual period or the date of conception. Pregnancy is conventionally counted as 40 weeks (280 days) from the last period, which is why that is the standard starting point. Alongside the due date, the calculator shows how far along the pregnancy is today and which of the three trimesters that falls in.
How to use it
Choose whether to base the estimate on the last period or the conception date, then pick that date. If you know your cycle length and it is not the average 28 days, adjust it so the estimate can be fine-tuned. The calculator returns the estimated due date, the current number of weeks and days pregnant, and the trimester. For example, a last period on 1 January gives a due date in early October.
The formula
The standard rule is Naegele’s rule: the due date is the first day of the last period plus 280 days, , which is 40 weeks. If a cycle is longer or shorter than 28 days, the estimate is shifted by the difference, since ovulation happens later or earlier. Dating from conception instead adds 266 days, because conception is about two weeks after the last period.
Where it is used
Expectant parents use it to plan for a baby’s arrival — booking leave, preparing the home and counting down the weeks. Midwives and doctors use the due date to schedule scans and antenatal appointments and to judge whether a pregnancy is progressing normally. It also frames the whole language of pregnancy, which is measured in weeks from the last period right up to the birth.