DueDateCalcFrequently Asked Questions
Straight answers about due date methods, accuracy and trimesters — with the uncertainty stated, not hidden.
How is my due date calculated from my last period?
The standard method is Naegele's rule: take the first day of your last menstrual period (LMP), add 280 days (40 weeks). If your cycle is usually longer or shorter than 28 days, this calculator adjusts by the difference — a 32-day cycle adds 4 days, for example.
How accurate is a due date calculator?
It is an estimate. Only around 5% of babies are born on their estimated due date, and most arrive within roughly a week either side. An ultrasound measurement in the first trimester is generally considered more precise, and your clinician's dating takes precedence.
What if I know my conception date instead?
Use the Conception tab. Pregnancy is typically counted as 266 days (38 weeks) from conception to the estimated due date, because the standard 280-day count assumes conception happens around day 14 of the cycle.
Can an ultrasound change my due date?
Yes. If an early ultrasound measurement suggests a meaningfully different gestational age than the LMP-based estimate, your provider may revise the due date. The ultrasound tab on this page lets you reproduce that style of calculation.
What does 'gestational age' mean?
Gestational age is how far along a pregnancy is measured from the first day of the last period — so it is about two weeks more than the time since conception. It is usually written as weeks plus days, such as 12 weeks, 3 days.
Is my data private?
Yes. Every calculation runs in your browser. The dates you enter are never sent to a server, stored, or shared.
Why did my due date change after my scan?
Because the scan measures the pregnancy directly. If an early ultrasound disagrees with the last-period estimate by more than a few days, many providers adopt the scan-based date. The last-period method assumes a 28-day cycle and day-14 ovulation, which are approximations.
What if my cycles are irregular?
Cycle-based estimates become less reliable. The conception method may help if you tracked ovulation; otherwise an early ultrasound is usually the anchor providers rely on. Mention irregular cycles at your first appointment.
What does it mean to be '12 weeks, 3 days'?
Gestational age is counted from the first day of the last period, in weeks plus remaining days. 12 weeks, 3 days means 87 days since that date — and about 73 days since conception, on the standard assumption.
When does each trimester end?
On the convention used here: the first trimester ends at 13 weeks, 6 days; the second at 27 weeks, 6 days; the third runs from 28 weeks to birth. Some sources round these to 14 and 28 weeks.
Is it normal to pass my due date?
Yes. The due date is the middle of a window, not a deadline. Many first pregnancies in particular run past 40 weeks, and providers typically add monitoring and discuss options as weeks 41–42 approach.