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Due Date Calculator

Reviewed by MD Katiuska Ríos Calderón

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How to Calculate the Due Date

With our due date calculator, you can easily obtain it based on the date of the last menstrual cycle.

In addition to the due date, you will also be able to obtain important information about the weeks of gestation.

So you know, if you want to calculate the probable due date in a simple, easy, and comfortable way, just enter the data requested by the calculator, press the button, and it will give you an approximate date of your delivery day.

Attention: you should always take the results obtained here as indicative, they can never replace or contradict your gynecologist's diagnoses.

Our content aims to inform you or introduce you to the calculation of the due date, it can be a point of reference, but do not take it as a certainty, since it is a very generalist method for a very individualized analysis, so we advise you to go to the relevant specialist.

How gestational weeks are calculated

Gestation is normally measured in weeks, a normal gestation period lasts 40 weeks, or 280 days. For this count, the first day of the last menstruation is considered the start and the delivery as the end of these 40 weeks of gestation. This is what we call gestational age.

This figure was proposed by Doctor Franz Naegele, a German obstetrician, who discovered that this way of calculating the due date was the closest to an exact result. That is why it is called Naegele's Rule or Formula.

The formula estimates the probable due date (FPP) based on the woman's last menstrual period (FUM).

To this date, we add one year, subtract 3 months, and then add 7 days. That is:

  • FPP = FUM + 7 days + 1 year - 3 months.

This approximation is based on the normal duration of a pregnancy, which lasts an average of 40 weeks (280 days) from the date of the last menstruation until the moment of the baby's birth.

Most women do not know the exact moment of conception, therefore, for practical reasons, it is counted from the first day of the last menstrual cycle.


How can you express the age of pregnancy?

Let's give an example, the tenth week of gestation means that 9 weeks and 1 to 7 days have already passed.

To write it in a technical way, you should use this model:

  • 9S3 = Nine weeks of gestation completed + 3
  • It can also be written = 10th SG

The pregnancy date can be expressed in weeks of gestation, in months, or in three phases, called trimesters.

Accuracy in calculating the probable due date

Statistically, most deliveries occur approximately on the calculated date, but only 4% do so on the exact date. This means that the remaining 96% did not occur on the day of the expected date.

It is very important to have an estimated due date, because if your baby arrives before that date, it would be considered a premature or preterm baby.

The American College of Obstetrics, along with the Society for Maternal-Fetal Medicine have reported new, more precise definitions for the period between 37 and 42 weeks of pregnancy "to reflect that babies born before 39 weeks have more health risks" (NCMHEP).

Deliveries that occur two weeks after and up to three weeks before the presumed due date are considered full-term, that is, approximately 90% of children are born within these parameters.

Babies that come into the world more than 3 weeks before this date are considered premature, and they represent 9% of births. Those born between week 37 and 39 are considered early term.

On the other hand, children who come into the world more than 2 weeks after the exact due date are considered late term.

This is much less frequent as statistics show that less than 1% of all births occur under these circumstances.

In any case, the calculation can be much more accurate if your gynecologist has the necessary information, which goes far beyond ultrasound and has to do with monitoring ovulation which has been followed from the start.

If all these data are known, the medical professional will know what your fertile days were and, therefore, when the baby will be born with greater precision by knowing some more data.

Calculating the probable day of delivery based on ovulation

The calculation is very simple. Take a regular cycle that lasts 28 days. Calculate, taking into account the first day of menstruation, what was the 14th day of the same, which is normally the day when ovulation tends to start.

During ovulation, the chances of becoming pregnant are higher, this occurs both on the day of ovulation and the four or five days before, since sperm have a lifespan of about 72 hours inside the female reproductive system.

Therefore, by calculating the approximate day on which your baby was conceived, you will be able to know much more accurately what will be the specific day of delivery. In this case, 266 days would be added to the date you have calculated.

The clinical estimation or calculation of gestational age, as well as the probable due date, are a reasonable approximation and can guide you and give you an idea of when your baby will be born. However, this estimation never replaces the obstetric evaluation and estimated date made by your doctor.

Before you go, we would like you to share this due date calculator on your social networks, maybe some of your contacts are interested. But, in any case, you would help us keep this community of free calculators alive.

Finally, if you find any deficiency in the programming or in the text, a proposal or you feel like encouraging us, do not hesitate to use the contact page, opinions are our fuel to continue with this project.

Bibliographic Resources

  • Ananth C. Menstrual versus clinical estimate of gestational age dating in the United States: temporal trends and variability in indices of perinatal outcomes. Paediatric and Perinatal Epidemiology. 2007;21(s2):22-30. Available at:
  • Engle W, American Academy of Pediatrics Committee on Fetus and Newborn. Age terminology during the perinatal period. 2004;114(5):1362-1364. Available at:
  • Let the baby determine the due date [Internet]. OCPL NIH. 2013. Available:
  • The American College of Obstetricians and Gynecologists Committee on Obstetric Practice Society for Maternal-Fetal Medicine. Definition of Term Pregnancy [Internet]. NIH. 2013. Available at:
  • Committee Opinion No. 561. Obstetrics & Gynecology. 2013;121(4):911-915. Available at:
  • Moldenhauer J. Premature Labor [Internet]. MSD Manual. Available at:
MD Katiuska Ríos Calderón
Doctor of Medicine and Surgery from UCLA. Specialized in Gynecology and Reproduction. Subspecialized in Endocrine Gynecology and Climacteric.
Dr. Katiuska Ríos Calderón holds a degree in Medicine from the Universidad Centr... Read more »

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