How To Calculate Your Due Date If You Don’t Remember When Your Last Period Was

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The second you see a positive pregnancy test result, it's only natural to start doing the math to figure out the delivery date. After all, it's handy to know how much time is left before the baby's arrival. But how can you calculate your due date if you don't remember when your last period occurred? There are some workarounds, thankfully.

Why is this missed period so important, anyway? For the most part, it makes the whole calculation process much easier. Because most pregnancies last for approximately 40 weeks, the easiest way to estimate the due date is to figure out the last menstrual period (LMP) and add 40 weeks to that, as noted in What to Expect. It's simple enough, but not everyone keeps detailed records of menstruation, and some people have irregular periods that are notoriously difficult to track.

There are plenty of other ways to guess a due date, however. “The best way to calculate an estimated due date when you don’t know the date of the last menstrual period is by ultrasound,” as doula Christine Strain, CD (DONA), tells Romper. “Early ultrasounds are one of the most accurate ways to date a pregnancy (even more so than LMP as ovulation dates can vary within a cycle).”

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Another way is to guess the baby’s conception date. “If the person knows the date of the sex that led to conception that would be another way to estimate it, but it could be off by several days as sperm can live for several days in the Fallopian tubes,” says Strain. Although not as exact as ultrasounds, it’s still a helpful tool to have in mind.

In addition to ultrasounds and conception date estimates, there are developmental milestones that can guess the baby's age, and from there estimate a due date. For example, dopplers can first detect a baby's heartbeat around 10 to 12 weeks, as explained in WebMD. Once that amazing milestone occurs, the healthcare provider can estimate a due date more effectively.

Also, it's possible to use fundal height measurement to guess the baby's approximate stage of development. Fundal height is the distance from the top of the uterus to the pubic bone, and it can play a neat role in pregnancy, as noted in the Mayo Clinic. At about halfway through the pregnancy, a fundal height measurement can be used to approximate the number of weeks a woman has been pregnant. A woman who is 29 weeks pregnant would have an expected fundal height measurement of around 29 centimeters, for example. The measurements may vary a little, but it's still an interesting way to estimate the baby's development, and from there the due date.

For the most part, though, there's no reason to stress about getting the perfect due date, because pregnancies are all a little different. Babies tend to arrive when they're ready, regardless of what ultrasounds, dopplers, or fundal height measurements predict. Although due dates can make it easier for the OB-GYN to ensure the baby is growing and developing properly, it's an estimate and not a deadline, as noted in Parents. “I try to not have moms focus on a due date but a due month rather! In most cases your baby will come when it’s ready!!” as birth and postpartum doula Liza Maltz tells Romper. In fact only about 5 percent of deliveries actually take place on their due date, as further noted by Parents. Wherever the pregnancy stands in the due date department, you'll just have to wait for the baby to arrive in his own time, like everyone else.


Christine Strain, certified doula

Liza Maltz, birth and postpartum doula

An earlier version of this post was published on 10/24/18. It was updated on 9/9/19.

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