Due date calculator: three ways to find your due date
Your due date anchors your whole pregnancy — every scan, screening, and milestone is scheduled around it. Here are the three standard ways to calculate it, and what to do once you know it.
Calculate your due date
Pick whichever date you have. Everything is worked out in your browser and nothing is sent anywhere.
Method 1: from your last menstrual period
The classic calculation (Naegele's rule): take the first day of your last period and add 280 days — exactly 40 weeks. This is the method most healthcare providers use by default, and it works well if your cycles are fairly regular.
Method 2: from your conception date
If you know when conception happened — for example through ovulation tracking or fertility treatment — add 38 weeks (266 days). The two-week difference versus the LMP method accounts for the time between your period and ovulation.
Method 3: from an ultrasound estimate
An early dating scan measures your baby directly and is often the most accurate estimate of all. If your provider gives you a due date from an ultrasound, use that one — it overrides the calendar math.
What a due date really means
Only about 1 in 20 babies arrives on the due date itself. Think of it as the middle of a window: anywhere from week 37 to week 42 is considered normal timing. The due date's real job is to keep your care on schedule — and to give you a number to count down to.
Calculate it once in Lunera, then never think about it again
Lunera supports all three calculation methods — last menstrual period, conception date, or a known due date. Pick whichever you have during a 30-second setup, and the app handles the rest:
- A live due date countdown ("136 days to go") on your home screen and widgets
- Your progress through all 40 weeks as a percentage
- Automatic trimester detection as the weeks pass
- Got a new estimate after a scan? Change the date in settings and everything recalculates
Quick answers
Can my due date change?
Yes — it's common for a first-trimester ultrasound to shift the estimate by a few days. Your provider will tell you if the official date changes; just update it in your tracker.
How likely is birth on the due date?
Around 5% of babies are born on their exact due date. Most arrive within a two-week window either side, which is why weeks 37–42 are considered term.
Track your pregnancy with Lunera
Week-by-week baby development, due date countdown, widgets, and a one-tap contraction timer — free, private, and subscription-free on iPhone.
This guide is for informational and educational purposes only and is not medical advice. Lunera is not a medical device. Always consult your doctor, midwife, or healthcare provider with any questions about your pregnancy or labor.