How Your Due Date Is Estimated
Most pregnancies last about 280 days — 40 weeks — counted from the first day of the last menstrual period (LMP), not from conception. This convention exists because the date of a period is usually known, while the exact day of ovulation and conception rarely is. The method is known as Naegele's rule, after the 19th-century German obstetrician Franz Naegele, and it remains the standard starting point for dating a pregnancy.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, ovulation usually happens later or earlier, so the calculator shifts the due date by the difference. A 32-day cycle, for example, moves the estimate four days later.
Only about 1 in 20 babies arrive on their due date
A due date is an estimate of the middle of a window, not an appointment. Most babies are born between 37 and 42 weeks, and only a small percentage — commonly quoted as around 4–5% — arrive on the exact day. It is more helpful to think of a "due month" centred on the date shown above.
Trimesters at a glance
| Trimester | Weeks | Highlights |
|---|---|---|
| First | 1 – 13 | Organs begin forming; nausea and fatigue are common |
| Second | 14 – 27 | Movement often felt; anatomy scan around 18–22 weeks |
| Third | 28 – birth | Rapid growth; preparing for birth |
Ultrasound dating
An early ultrasound — especially one in the first trimester — measures the baby directly and is generally considered the most accurate way to date a pregnancy. If the ultrasound estimate differs significantly from the LMP estimate, your care provider may update your due date. Pregnancies conceived through IVF are dated from the embryo transfer date instead.
Pregnancy weeks vs. baby's age
Because pregnancy is counted from the last period, the "gestational age" is about two weeks more than the baby's actual age since conception. At 10 weeks pregnant, the embryo has been developing for roughly 8 weeks. Once your baby is born, our age in days and age in months calculators are perfect for tracking those early milestones.
Frequently Asked Questions
How accurate is a due date calculator?
It gives the same estimate your clinician would calculate from your last period. Accuracy depends on knowing the date of your period and on cycle regularity. An early ultrasound is generally more precise.
What if I don't know the date of my last period?
Your healthcare provider can date the pregnancy with an ultrasound, which measures the size of the embryo or fetus.
Why is pregnancy counted from my last period and not conception?
The first day of a period is a date most people know, whereas ovulation and conception dates are rarely certain. Counting from the period gives everyone a consistent reference point.
Can my due date change?
Yes. If an ultrasound suggests the pregnancy is further along or less advanced than the period-based estimate, your provider may revise the date — most commonly after a first-trimester scan.
Disclaimer: This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Formulas are population averages and may not fit your individual situation. Always consult a qualified healthcare provider about your health.
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