Due Date Calculator

A due date calculator based on LMP is about 90% accurate within a two-week window. For example, if your LMP was January 1, your due date is October 8 (40 weeks later). However, only 4% of babies are born on that exact date. Factors like irregular cycles, unknown conception date, and ultrasound measurements can shift the date. An early ultrasound (before 14 weeks) is the most accurate method, with a margin of error of ±5-7 days. If your cycle is 35 days, add 7 days to the LMP-based due date. For a personalized estimate, use our due date calculator and share the result with your provider.

The standard due date calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is 32 days, ovulation likely occurs around day 18, so add 4 days to the due date. For a 24-day cycle, subtract 4 days. For example, with a 35-day cycle and LMP of March 1, the unadjusted due date is December 6; adjusted, it becomes December 13. This adjustment matters because it can change the timing of prenatal tests like the 20-week anatomy scan. Our calculator lets you input your cycle length for a more precise estimate. Always confirm with an ultrasound if available.

Yes. If you know your conception date—perhaps from IVF or a single intercourse date—add 266 days (38 weeks) to get your due date. For instance, if conception occurred on June 1, your due date is March 4. This method is more accurate than LMP if you have irregular cycles or know the exact date. However, conception can occur up to 5 days after intercourse, so the date is still an estimate. Our calculator allows you to switch between LMP and conception methods. For IVF, the due date is calculated from the embryo transfer date: add 266 days for a day-3 transfer or 263 days for a day-5 transfer.

Gestational age is measured from the first day of your last menstrual period, while fetal age is measured from conception. Gestational age is typically two weeks ahead of fetal age. For example, when you are 10 weeks pregnant by gestational age, the fetus is about 8 weeks old. This distinction matters because all prenatal milestones, like when you can hear the heartbeat (around 8-10 weeks gestational age) or when screening tests are done, use gestational age. Our calculator shows gestational age in weeks and days. If you want to know fetal age, subtract two weeks from the gestational age. Keep in mind that fetal age is rarely used in clinical settings.

A due date is a single estimated day (40 weeks from LMP), but a delivery window is the range when birth is considered full-term. Full-term is defined as 39 to 40 weeks and 6 days. For example, if your due date is October 8, your full-term window is September 17 to October 8. Babies born before 37 weeks are preterm, and after 42 weeks are post-term. Only about 10% of babies are born on their due date, and 80% are born within two weeks before or after. Your provider may schedule induction if you reach 41-42 weeks. Use our due date calculator to find your due date, then mark your calendar for the 37-week mark.

If your periods are irregular, an LMP-based due date may be off by a week or more. The best approach is to use an early ultrasound (between 8 and 13 weeks) to date the pregnancy. For example, if your LMP suggests a due date of May 15 but an ultrasound at 10 weeks shows a due date of May 22, the ultrasound date is more reliable. If you don't have an ultrasound, use the average of your last three cycle lengths. For instance, if your cycles range from 30 to 40 days, average them to 35 days and add 7 days to the standard due date. Our calculator allows you to input your cycle length, but it can't account for irregularity. Always consult your provider for the most accurate dating.