The second you see a positive pregnancy test result, it's only natural to start doing the math to figure out your due date. After all, it's handy to know how much time is left before the baby's arrival, and you probably want to know just how far along you are in your pregnancy. But how can you calculate your due date if you can’t remember your last period? There are ways to figure out how far along you are without knowing your last period, thankfully.
How to determine your due date if you can’t remember your last period
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 pregnant person’s last menstrual period and add 40 weeks to that. It's a simple enough calculation, 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 calculate a due date, however. If you can’t remember your last period, don’t worry. You are not the first pregnant person to experience this. “The best way to calculate an estimated due date when you don’t know the date of the last menstrual period is by ultrasound,” says Christine Strain, a certified birth doula. “Early ultrasounds are one of the most accurate ways to date a pregnancy (even more so than a last menstrual period as ovulation dates can vary within a cycle).”
More ways to calculate your due date if you can’t remember your last period
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.
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. 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. “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. 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, a certified birth doula
Liza Maltz, postpartum doula
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