Pregnancy Due Date Calculator
Estimate your due date and current gestational age based on the first day of your last menstrual period (LMP). Uses Naegele\u2019s rule (LMP + 280 days).
How the math works
Naegele\u2019s rule, formulated in the early 19th century by German obstetrician Franz Karl Naegele, estimates the expected day of delivery as the first day of the last menstrual period (LMP) plus 280 days, or equivalently 40 weeks. It assumes a regular 28-day cycle with ovulation on day 14.
If your cycle length differs from 28 days, the rule can be adjusted: shift the due date forward or backward by the difference. A 30-day cycle, for example, suggests adding two days to the standard estimate. Our calculator does this automatically when you change the cycle length.
Trimesters at a glance
- First trimester: weeks 1–13 (organogenesis, highest miscarriage risk window).
- Second trimester: weeks 14–27 (the "honeymoon" trimester for most people).
- Third trimester: weeks 28–40+ (rapid fetal growth, third-trimester screening).
What "due date" really means
Your due date is the centre of a probability distribution, not a deadline. Roughly 4% of babies arrive on the predicted date; about 80% arrive within ten days on either side. Your healthcare provider may revise the date after an early ultrasound, which is a more reliable form of dating in the first trimester.
This calculator is for general information. It does not replace professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
Frequently asked questions
How is the due date estimated?
We use Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. This is the standard convention used by obstetric professionals worldwide and matches roughly 4–5% of actual delivery dates exactly, with the majority of births occurring within two weeks on either side.
How accurate is Naegele’s rule?
It is an estimate, not a prediction. Studies show that fewer than 5% of babies are born on their predicted due date; about 80% arrive within ten days of it. The number is most useful as a milestone for prenatal scheduling, not as a calendar reservation.
What if my cycle is not 28 days?
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ask your provider about an adjusted due date, or use an early ultrasound for the most reliable dating.
What is gestational age vs. fetal age?
Gestational age is counted from the first day of the last menstrual period. Fetal age is counted from conception, which is roughly two weeks later. Clinical practice almost always reports gestational age.
Should I rely on this calculator for medical decisions?
No. This is an information tool. Pregnancy dating, particularly when planning for delivery, induction, or screening, should always be confirmed by a qualified healthcare professional.