Due Date Calculator

It is a useful estimate, not a precise prediction. Only about 4-5% of babies are born on their exact due date, and most arrive in the weeks around it rather than on the day itself. A due date from your last menstrual period (LMP) relies on Naegele's rule and assumes a regular 28-day cycle, so it carries real uncertainty. First-trimester ultrasound, which measures the embryo directly, is more accurate and is the dating method your provider will usually prefer. Think of the due date as the midpoint of a normal delivery window, roughly 37-42 weeks, rather than a deadline. For your individual timeline, rely on your healthcare provider's dating.

Your LMP-based due date will be less accurate, and early ultrasound dating becomes especially valuable. Naegele's rule assumes a regular 28-day cycle with ovulation around day 14, so when cycles are longer, shorter, or unpredictable, the estimated date can drift. Someone with a 35-day cycle, for example, tends to ovulate nearer day 21, which can place the true due date about a week later than the LMP calculation suggests. A first-trimester ultrasound sidesteps this problem because it measures the embryo directly instead of relying on cycle assumptions. If your periods are irregular or you are unsure of your LMP, mention it to your healthcare provider so dating can be confirmed by scan.

Usually because a first-trimester ultrasound disagrees with your LMP-based date by more than about 7 days. The scan measures the embryo directly, which makes it more reliable than a calculation built on the date of your last period and an assumed cycle length. When the two differ enough, providers generally update the due date to match the early ultrasound, following guidance from the American College of Obstetricians and Gynecologists (ACOG). Later scans are less precise for dating because babies grow at different rates after the first trimester. Your date may also be revised if you have a confirmed conception date from fertility treatment. Any change is routine, and your provider can explain the reason.

It is common and, for the first several days, generally not a cause for concern. Because the due date is a midpoint rather than a deadline, many pregnancies continue past 40 weeks. Providers typically keep a closer eye on things after the due date, with extra monitoring of the baby's wellbeing, and will discuss induction of labor as you approach 41 to 42 weeks. A pregnancy that reaches 42 weeks is called post-term, and care teams watch for issues such as reduced amniotic fluid. Recommendations vary by individual, so the timing of any monitoring or induction is a decision to make together with your healthcare provider based on your specific situation.

Healthcare providers measure pregnancy in weeks because it is more precise for tracking development and scheduling tests. Gestational age is counted from the first day of your last menstrual period, which is why a full-term pregnancy is described as 40 weeks even though conception happens about two weeks into that count. Months are only an approximation: dividing weeks by about 4.3 gives a rough month, but pregnancy months never line up neatly with calendar months. At 16 weeks you are around 4 months along, and at 40 weeks you have completed roughly 9 calendar months plus a few days. That mismatch is exactly why clinicians stick with weeks for accuracy.