Pregnancy Due Date Calculator

Estimate your due date and how far along you are based on your last menstrual period or conception date. Results are estimates, not medical advice.

How Due Dates Are Calculated

Pregnancy due dates are estimated using Naegele's rule, which adds 280 days or 40 weeks to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation occurring on day 14. Our calculator also adjusts for different cycle lengths, as women with longer cycles typically ovulate later and may have later due dates. The estimated due date represents the midpoint of a normal delivery window, as only about 5 percent of babies are born on their exact due date.

Understanding Pregnancy Trimesters

Pregnancy is divided into three trimesters of approximately 13 weeks each. The first trimester from weeks 1 to 12 involves critical organ development. The second trimester from weeks 13 to 26 brings rapid growth and typically improved maternal comfort. The third trimester from weeks 27 to 40 focuses on fetal weight gain and lung maturation. Our calculator shows your current trimester, weeks remaining, and key developmental milestones for each stage.

Important Considerations

Due date calculations are estimates that may be refined by ultrasound measurements, particularly in the first trimester when dating is most accurate. Full-term delivery ranges from 37 to 42 weeks. Factors including maternal age, health conditions, and pregnancy history can affect timing. Always consult your healthcare provider for personalized guidance. This calculator is for informational purposes only and should not replace professional prenatal care or medical advice.

Accuracy of Due Date Estimates

Due date calculations are estimates with a margin of uncertainty. Only about 5 percent of babies arrive on their exact due date. The normal delivery window spans from 37 to 42 weeks of gestation. First-time mothers tend to deliver slightly later than the estimated date on average. Ultrasound dating in the first trimester is accurate to within plus or minus 5 to 7 days, while last menstrual period calculations assume regular 28-day cycles and may be less accurate for women with irregular periods. Your healthcare provider may adjust the due date based on ultrasound measurements.

Frequently Asked Questions

How is my due date calculated?

The most common method estimates your due date as 280 days, or about 40 weeks, from the first day of your last menstrual period, a convention known as Naegele's rule. This assumes a regular 28-day cycle with ovulation around day 14. If you know your conception date, the calculator can instead count roughly 266 days forward. Because cycles and ovulation timing vary, the due date is an estimate of the middle of a likely delivery window rather than a fixed appointment, and most babies arrive in the weeks around it.

How accurate is a due date estimate?

A due date is a best estimate, not a precise prediction. Only a small minority of babies are born exactly on their due date, and a full-term delivery is considered normal anywhere from about three weeks before to two weeks after it. Factors such as cycle length, time of ovulation, and individual variation all affect timing. Early ultrasound dating is generally more accurate than calculation from the last period, so your healthcare provider may adjust the estimate.

What are the three trimesters of pregnancy?

Pregnancy is conventionally divided into three trimesters. The first runs from week 1 to around week 12 and involves early development; the second spans roughly weeks 13 to 27 and is often when many people feel most comfortable; and the third covers about week 28 to birth, as the baby grows rapidly in preparation for delivery. The calculator shows which trimester an estimated date falls in to help you follow your progress.

Can I use this calculator if my cycle is irregular?

You can, but the result will be less reliable. The standard calculation assumes a regular 28-day cycle, so if your cycles are longer, shorter, or unpredictable, the estimated date may be off. If you know your conception or ovulation date, using that will improve the estimate. For irregular cycles, an early ultrasound arranged by your healthcare provider is the most dependable way to date the pregnancy.

Is this calculator a substitute for medical care?

No. This tool provides general estimates for informational purposes only and does not replace professional medical advice, diagnosis, or monitoring. Pregnancy should be overseen by a qualified healthcare provider who can confirm dating, track development, and address any concerns. Always consult your doctor or midwife for guidance specific to your situation.