Due Date Calculator
Estimate when your baby will arrive based on your last period or conception date.
How Due Date Calculation Works
Finding out you are expecting is one of life's most exciting moments, and the very next question is almost always: "When is the baby due?" Our due date calculator gives you an instant estimate so you can start planning, dreaming, and counting down the days.
The calculator uses Naegele's rule, the same method your doctor uses at your first prenatal appointment. It takes the first day of your last menstrual period (LMP) and adds 280 days — or 40 weeks — to arrive at your estimated due date. If you know the date of conception instead, we add 266 days (38 weeks), since conception typically occurs about 2 weeks after the start of your last period.
What the Results Tell You
Beyond the estimated due date itself, our calculator shows you how far along you are in weeks and days, which trimester you are currently in, and how many days remain until your due date. The visual progress bar gives you an at-a-glance view of your pregnancy journey from the first trimester through the third.
If you entered your last period date, the calculator also estimates your likely conception date. This can be helpful for understanding your timeline and for conversations with your healthcare provider.
Keep in Mind
A due date is exactly that — an estimate. Full-term pregnancy spans from 37 to 42 weeks, and only about 1 in 20 babies arrives on the predicted date. Think of your due date as the center of a window rather than a precise target. Factors like first-time pregnancy, family history, and your individual cycle length can all influence when labor begins naturally.
Your healthcare provider may refine your due date after an early ultrasound, which measures the baby directly and can sometimes be more accurate than calculations based on your last period. Always follow your provider's guidance and attend regular prenatal checkups throughout your pregnancy.
Frequently Asked Questions
How is a due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation occurring on day 14. If you know your conception date, the due date is 266 days (38 weeks) from conception.
How accurate is a due date calculator?
A due date is an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. Most full-term babies arrive within a window of 2 weeks before or after the estimated date. Your healthcare provider may adjust your due date based on early ultrasound measurements, which can be more precise than LMP-based calculations.
What if my cycle is longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle. If your cycle is consistently longer (say 35 days), you likely ovulated later, meaning your due date would be pushed back by about a week. Conversely, shorter cycles may mean an earlier due date. Discuss your cycle length with your healthcare provider for a more personalized estimate.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period — it is the standard 40-week timeline doctors use. Fetal age (also called embryonic age) is counted from conception, which is typically about 2 weeks less than gestational age. When people say pregnancy is '9 months,' they are roughly describing the 38 weeks of fetal age.
Can my due date change during pregnancy?
Yes. An early ultrasound (usually between 8-12 weeks) provides a measurement of the baby that can refine the due date. If the ultrasound-based estimate differs significantly from the LMP-based date (typically by more than 5-7 days), your provider may adjust the official due date.