Determining a due date isn't an exact science. In fact, your original due date is an educated guess based on the date of your last menstrual period (LMP) — or your embryo transfer date if you conceived via in vitro fertilization (IVF). As your pregnancy progresses and your health care practitioner gathers more information through ultrasounds and other indicators, your due date may be adjusted.

Learn why that first due date might have been off and when it might be adjusted so you can be prepared for possible changes to your delivery plans.

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Key Takeaways
  • Due dates are estimates, not exact predictions. Most due dates are initially calculated using the first day of your last menstrual period (or embryo transfer date for IVF pregnancies), but this method assumes a standard 28-day cycle and can be inaccurate.
  • Your due date may change as more information becomes available. Early ultrasounds, fundal height measurements, and certain blood test results can lead your health care practitioner to adjust your due date to better reflect your baby's development and gestational age.
  • A revised due date usually doesn't change your prenatal care. Even if your due date is adjusted, your prenatal appointments, screenings, and overall care plan will typically continue as scheduled. The change mainly helps providers better track your baby's development and determine the most accurate timeline for your pregnancy.

How are due dates usually calculated?

Your due date is typically calculated from the first day of your last menstrual period (LMP). Since most pregnancies last an average of 280 days, or 40 weeks, your due date is estimated by counting forward from that date.

However, this method assumes you have a regular 28-day menstrual cycle and ovulate on day 14. It doesn't account for irregular cycles or variations in the timing of your ovulation, so it's not always precise.

If you conceived using IVF, your due date is calculated using your embryo transfer date, taking into account whether a three- or five-day embryo was transferred.

Regardless of how you conceived, remember that your due date is only an estimate. Every pregnancy is different. Some babies arrive early, and others take their own sweet time and show up late.

Curious about your own due date — and whether it could change? The What to Expect app does the math for you and keeps track as your pregnancy progresses.

Why might my due date change?

Unless you conceived through IVF — or know for sure which roll in the hay was the lucky one — your practitioner has no way of knowing the exact time of conception. However, a first trimester ultrasound can help confirm or adjust your pregnancy timeline.

Here are some common reasons your baby's arrival day might change:

  • You have irregular periods. Does your monthly go MIA from time to time? It can be pretty difficult to determine a due date using the date-of-last-period method for women with irregular cycles — which is why an ultrasound exam is often required to determine gestational age. "My cycles were not 28 days, and I ovulate later than day 14, so I knew using my LMP would be off," says What to Expect Community mom l1996.
  • Your first date relied on Doppler. Many pregnancies are initially confirmed with a Doppler heartbeat monitor rather than an ultrasound. If that's the case, your original due date may have been less than accurate. An early ultrasound, usually done between 6 and 9 weeks, provides one of the most accurate estimates by measuring the embryo or fetus.
  • Your first ultrasound was in the second trimester. First-trimester ultrasounds are more accurate for dating a pregnancy than second-trimester scans, according to the American College of Obstetrics and Gynecology (ACOG).[1] If your first ultrasound occurred later, your due date may be revised. "My due date was set at my 8-week scan," says Community mom scoopstroopx5, who was breastfeeding and had no period on which to base a date.
  • Your fundal height is above average. At each appointment, you'll have a fundal height measurement, which is the distance from the top of your pubic bone to the top of your uterus. This number generally corresponds to how far along you are. If it differs significantly from expected growth patterns, your practitioner may adjust your due date. "All of my due dates have changed every pregnancy," says Community mom pixelstyx, who is having her fourth baby. "They adjusted all my dates based on baby measurements."
  • You have abnormal AFP levels. Between weeks 14 and 22, your practitioner may order a blood test to screen for alpha fetoprotein (AFP), a protein produced by your baby's liver and yolk sac. (This is often part of the quad screen.) AFP levels vary widely throughout pregnancy. While high levels of AFP sometimes indicate a potential genetic abnormality, one of the most common explanations is that your due date was miscalculated.

How does a changed due date impact my prenatal care?

A change in your due date will typically have little effect on prenatal care. You'll continue attending your scheduled appointments, screenings, and checkups as usual. Be sure to discuss any concerns or questions with your practitioner along the way.

If your due date changes, try not to let it throw you off balance. Even with the most accurate calculations, there's no way to know exactly when labor will begin, and whatever day your baby arrives will be special.

Enjoy the element of surprise — it's great practice for parenting!