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Pregnancy Due Date Calculator Guide
Estimate a pregnancy due date from the first day of the last menstrual period (LMP), using Naegele's rule. The calculator also shows how far along the pregnancy is and which trimester it falls in.
Due date ≈ first day of last period + 280 days (40 weeks). If your cycle isn't 28 days, the estimate shifts by the difference from 28.
How Is a Due Date Calculated?
Pregnancy is counted from the first day of the last period, and full term is about 40 weeks. For a last period starting January 1, the estimated due date is around October 8. Only about 1 in 20 babies arrive exactly on the due date.
Trimesters and Accuracy
The first trimester runs through week 13, the second through week 27, and the third to birth. An ultrasound, especially in the first trimester, often gives a more accurate date than LMP alone. This tool is a general estimate and not a substitute for medical care.
This is a general estimate for information only, not medical or nutritional advice. Individual needs vary with health, goals, and other factors — talk to a doctor or registered dietitian for guidance specific to you.
Related: Date Difference, Days Until, and Age Calculator.
Related calculators: BMI Calculator, Ideal Weight Calculator, BMR Calculator.
Frequently Asked Questions
How is a due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period, known as Naegele's rule.
How accurate is a due date?
It's an estimate — only about 5% of babies are born on the exact date, and most arrive within a couple of weeks either side.
What if my cycle isn't 28 days?
The calculator adjusts the estimate by the difference between your cycle length and 28 days.
Is an ultrasound more accurate?
Often yes, especially early in pregnancy, which is why doctors may adjust the date after a scan.
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