Pregnancy Due Date – How To Calculate It?

You just found out you’re pregnant? Congratulations! After the first celebratory moments, perhaps you’ll want to know when to expect your bundle of joy. There are dozens of pregnancy due date calculators, but figuring out the date on your own is ridiculously easy.

Before telling you how to calculate it, remember that the pregnancy due date is an estimate. No calculator or method can tell you exactly when you’ll go into labor. Moreover, the due date can change.

That said, here’s how to get an estimate of when your baby will arrive.


There are two methods to calculate your pregnancy due date. The Naegele’s method is the simplest, although for some women the result could be inaccurate. The Mittendorf-Williams method is more accurate but it involves a complex algorithm.

Since most online calculators figure out your due date based on the Naegele’s method, we’re going to start with it.


This is the simplest method to calculate your due date. The method was established by a German obstetrician in the early nineteenth century and bases the calculation on a perfect 28-day cycle with ovulation taking place on day 14.

Based on this assumption, you’ll deliver your baby after 280 days, or nine months and one week, from the first day of your last period.

Therefore, to calculate your pregnancy due date, add nine months and 7 days to the first day of your last period. For example, if the first day of your last period was April, 1, your pregnancy due date is January, 8 the following year.

However, most women don’t have perfect cycles and ovulation doesn’t always occur on day 14. For this reason, the due date obtained through this method is only a rough estimate.


According to a study conducted in 1990 by Mittendorf and Williams, the Naegele’s rule isn’t exactly accurate and that the actual term for labor is with 2-4 days longer. Their research showed that the average due date for first-time moms is 288 days. This figure drops to 283 days for multiparas, which is still longer than the 280 days predicted by Naegele.

As a result, Mittendorf and Williams came up with their own algorithm that suggests counting back three months from the first day of the last menstrual period, then add 15 days for the first baby or 10 days for the subsequent pregnancies.

The study was conducted on Caucasian women and since then, scientists adapted this method to all other races.

Today, the Mittendorf-Williams algorithm includes complex calculations that consider not only the number of pregnancies and the first day of the last menstruation but also the race, height and weight, alcohol drinking habits and the average length of the cycle.


Due date calculators can only calculate an estimate. Not only each woman and each pregnancy is unique but the pregnancy itself can last anywhere between 38 and 42 weeks. Imprecision in calculations also results from an irregular cycle.

Because ovulation doesn’t always occur at regular intervals, it is sometimes impossible to foresee the exact fertilization date. The size of the uterus and that of the embryo can provide rough indications on these dates but they are not always accurate.

That’s why obstetricians prefer to talk about pregnancy due month rather than due date. In fact, only 4% of the babies are born on their due date and all the others are born either in the two weeks prior or in the two weeks after the stated due date.


First of all, there is to say that the due date you calculated and the due date calculated by your practitioner may not coincide in the first place. This happens because practitioners and obstetricians often base their calculations on the ultrasonography results or on blood tests that detect the alpha-fetoprotein levels.

Irregular periods, a too small or too large fetus, an abnormal fundal height and many other variables can cause calculation errors that place the due date either too early or too late.

For this reason, practitioners talk about due date estimates and adjust this figure based on the progress of your pregnancy.

Therefore, while your due date can change, that’s not a reason for concern. Also, some babies can be born after the 42-week threshold. But remember, as long as your obstetrician is not alarmed, it means your baby is developing normally and you shouldn’t worry.


Besides the mathematical ways described above, physicians also use alternative methods to determine your pregnancy due date. Using these methods at home is impossible; however, prenatal care is crucial for ensuring a healthy development of the baby in the womb, therefore you can just ask your practitioner to use them to help you confirm the due date.


Ultrasonography is a non-invasive procedure that helps doctors see what’s going on inside your womb; this procedure is also one of the most accurate in establishing the due date. The earlier you get the first ultrasound the better because your practitioner will be able to establish the date you conceived with more accuracy.

Although this procedure is performed routinely in many cases, some practitioners can only recommend an early ultrasound if the pregnancy is considered at risk. In fact, your doctor may only perform a routine early ultrasound if you’re over 35, have had miscarriages in the past, or you’re facing pregnancy complications due to your medical history.

However, your physician can’t deny you an early ultrasound if you ask for one. So, if you want to check the accuracy of your calculations, ask your physician to perform an early ultrasound.


Less accurate than an early ultrasound, pregnancy milestones also offer indications on whether your pregnancy due date is correct or not. One of them is the fetal heartbeat which is heard for the first time at about 9 to 12 weeks.

As it’s easy to understand, this milestone indicates the due date with a three-week inaccuracy but fetal measurements can help the doctor understand how many weeks pregnant you are. The same goes for the first feel of the fetal movement, which happens between the 16th and the 22nd week.

When you actually feel the first movement depends on how many pregnancies you had; other factors can also influence it.


The fundal height refers to the top of your uterus that should reach your navel at about week 20. Your practitioner uses this measurement to determine your gestational age, and you can also use this figure to confirm the due date.

Yet again, the figure is only indicative. Some women have a higher fundal height and in this case, it may happen to give birth after the “traditional” 42 weeks. This only happens because of a miscalculation of your gestational age, which means your baby is still born on term.


Along with the fundal height, the size of your uterus also offers important indications about your gestational age and due date. Practitioners tend to measure the size of the uterus at each prenatal check, and standard measurements allow you to use these number to determine the gestational age, thus the due date.

Yet again, the figure is inaccurate and it depends on your morphology.


Calculating an accurate pregnancy due date is often impossible, but there are many medical checks and tests you can use to verify the accuracy of your calculations. Even if you have an accurate due date, remember that most babies are born two weeks before or after the estimated date, therefore you shouldn’t worry if your bundle of joy decides to meet you sooner or later your due date.

A normal pregnancy can last anywhere between 38 and 42 weeks; an irregular period or a miscalculation of the date you conceived can easily mess your results. Moreover, your physician can change and adjust the due date estimate based on the progress of the gestation.

So, instead of worrying about when your baby is due, enjoy your pregnancy and get ready for the baby to come.

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