Pregnancy Due Date Calculator
Estimate your due date, how far along you are, and your trimester.
Formula
About this calculator
A pregnancy due date calculator estimates when your baby is likely to arrive from a single, easy-to-remember date: the first day of your last menstrual period (LMP). It uses Naegele's rule, the standard clinical method, which adds 280 days — exactly 40 weeks — to that date. Because a pregnancy is dated from the LMP rather than conception, the count already includes the roughly two weeks before you actually conceived.
The default assumes a regular 28-day cycle, with ovulation and conception around day 14. If your cycles are consistently longer or shorter, the calculator shifts the estimate by the difference — a 32-day cycle pushes the due date about four days later, for example. Alongside the due date it shows how far along you are in weeks and days, your current trimester, and how many days remain.
Trimesters are counted from the LMP: the first runs through week 13, the second from weeks 14 to 27, and the third from week 28 to birth. These are estimates, not appointments — only about one baby in twenty arrives on the exact due date, and most are born within two weeks either side. An early ultrasound gives a more precise date, which your provider may use in place of the LMP calculation.
Frequently asked questions
How is the due date calculated?
It uses Naegele's rule: 280 days (40 weeks) added to the first day of your last period, with an adjustment if your cycle is longer or shorter than 28 days.
Why start from my last period and not conception?
The date of conception is rarely known exactly, but the last menstrual period usually is. Dating from the LMP is the standard clinical convention, so pregnancy weeks include roughly the two weeks before conception.
How accurate is the estimated due date?
It is an estimate. Only about 5% of babies arrive on the exact due date; most are born within two weeks before or after. An early dating ultrasound is more precise.
Does cycle length really matter?
Yes. If you ovulate later than day 14 because your cycle is long, conception is later and the due date moves back. This calculator adjusts by the difference between your cycle length and 28 days.
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⚠️ This is an estimate for general information only and is not medical advice. Always confirm dates and pregnancy care with your doctor or midwife.