If the pregnancy test result appears positive, monitoring the pregnancy month by month is very important, so that the development of the fetus can be monitored and the due date can be estimated. Here comes the role of the pregnancy calculator, which is a tool that predicts the date of birth through some information, such as: the date of the last menstrual period, the number of days of menstruation, and other information. Through the article, we will focus on how to use the pregnancy calculator, how accurate it is in calculating the expected date of birth, and how the age of the fetus can be determined. 

What is a pregnancy calculator?

A pregnancy calculator is a simple tool that helps mothers calculate their pregnancy weekly, monthly, and expected due date. It is usually a web page or application that contains:

• A box for entering the date and a calendar for manually writing the date.
• Button to calculate the month and show the result “View Result”
• The result appears like this: “You are one week……of pregnancy.”
• Some applications have additional tools such as a table with details of the week, tips for pregnant women, and the ability to save the result. 

 How to use the pregnancy calculator?  

The steps for using the pregnancy calculator are very simple and easy, and you can use it on your own without assistance by doing the following:

1. Open the account for the calculator, whether on a website or mobile application.
2. Start by entering the date of the first day of the last menstrual period. Be careful when entering the date.
3. Determine and choose whether your cycle is regular or not, as some calculators request this information.
4. After ensuring that the date and information entered are correct, click on the “Calculate” or “View Result” button.
5. Wait a little while until you see the exact result, and it usually says, “You are a week…”
6. The calculator may provide more information about your expected due date and the stages of fetal development in a complete and simple table.

Learn about: Everything you want to know about pregnancy symptoms in the first week

How can the due date be calculated?

When visiting the doctor for the first time after a pregnancy test, he will ask about the date of the pregnant woman's last menstrual period, and the usual number of days in the cycle. He may also ask about the dates of intercourse in the previous month, so that he can, from these dates, calculate the duration of pregnancy and the expected date of birth. 

The expected due date is the date on which spontaneous labor begins, and it can be calculated in a simple way known as “Nagle’s rule for calculating pregnancy,” which can be applied as follows: 

• Add 7 days to the date of the first day of your last menstrual period. 
• Subtract three months from the resulting date. 
• Change the year according to the year of the current pregnancy. 

For example, if the date of your last menstrual period is January 1, 2024, the expected due date can be calculated using Nagle's rule as follows: 

January 1 + 7 days = January 8. 

January 8, 2024 - three months = October 8, 2023. 

So, by changing the year to match the current year, the expected date of birth according to previous calculations becomes October 8, 2024. 

The accuracy of this method depends on the accuracy of the dates that the mother remembers, and whether her menstrual cycle is regular and lasts 28 days. However, in cases where the date of pregnancy is precisely known, as is the case with ICSI, the expected date of birth is calculated by adding 266 days to the date of fertilization. 

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Is the pregnancy calculator correct? 

Although the pregnancy calculator is widely used and easy to use, it may not be very accurate if the woman's menstrual cycle is irregular. Also, the date of birth may vary from one woman to another according to other criteria such as whether it is the woman's first pregnancy or not, whether she is pregnant with twins, and other factors. 

A pregnancy calculator may also not be able to be used if a woman forgets the date of her last menstrual period, and does not know the length of her menstrual cycle (the days between each two cycles). Here, an ultrasound examination is the method used to measure the age of the fetus and the expected date of birth.

احجز استشارتك في مركز صحة المرأة At Al Mousa Specialist Hospital

Pregnancy calculator in weeks 

Many online pregnancy calculators and phone applications provide a pregnancy calculation by weeks, the development of the fetus each week, the symptoms that the mother may experience, and tips to overcome them and maintain a healthy pregnancy. 

Because the exact date of fertilization is not known precisely, pregnancy in weeks is calculated as approximately 40 weeks (280 days) from the date of the first day of the last menstrual period, or 38 weeks from the date of fertilization, which is known precisely in cases of ICSI. 

It is worth noting that only 30% of pregnancies reach the fortieth week of pregnancy, and in the rest of the cases birth occurs before that. 

Learn about: How to calculate ovulation days and chances of pregnancy

Birth calculator in months 

It is known that the number of months of pregnancy is approximately nine months, and calculating pregnancy in months is the common method for women, as it is easier for them to say that their pregnancy is in the fourth month instead of the fourteenth week. 

Pregnancy in months can be calculated by adding 280 days (9 months and 7 days) to the date of the first day of the last menstrual period. 

For example, if the date of the first day of the last menstrual period is January 1, 2024, then the expected date of birth is: October 8, 2024. 

Pregnancy is usually divided by months into trimesters (three trimesters), so each trimester is called a trimester, so we have the first trimester, the second trimester, and the last trimester, and one trimester of pregnancy includes approximately three months. 

Pregnancy can be calculated by seasons, weeks and months as follows: 

• First trimester of pregnancy: It lasts for three months, starting from the first day of the last menstrual period, and in weeks, it ends by the thirteenth week of pregnancy.

• Second trimester of pregnancy: It lasts from the beginning of the fourth month until the end of the sixth month, that is, from the fourteenth week to the twenty-seventh week of pregnancy. 
• Third trimester of pregnancy: It extends from the seventh month until the end of the ninth month, which corresponds to the weeks from the twenty-eighth week to the fortieth week of pregnancy. 

Hijri pregnancy calculation

There are some applications that support calculating pregnancy in the Hijri, so the woman enters the date of the first day of the last menstrual period, but in the Hijri calendar, for example, “Muharram 10, 1446 AH.” The electronic pregnancy calculator converts that date to calculate the gestational age at that moment, and the result shows the week of pregnancy and the date of birth in the Hijri and Gregorian calendar as well.

What is the difference between the Hijri and Gregorian pregnancy calculator? 

The difference between the Hijri pregnancy calculator, which deals with lunar days, and the Gregorian calendar calculator includes that the Hijri calendar calculator has a different number of days in the year than the Gregorian calendar, so it specifies a date of birth that is different from the Gregorian one. Secondly, the Gregorian pregnancy calculator is the most widely used around the world and in Arab countries, so the possibility of a Hijri calendar calculator application being available is less.

Pregnancy calculator from the day of fertilization 

Some pregnancy calculators need to enter the date of fertilization so that the expected date of birth can be determined accurately, and in fact, calculating the date of birth based on the date of fertilization is more accurate. 

The likely due date can also be calculated manually using the fertilization date, by adding 266 days or 38 weeks to it. 

The problem with this method is that some women do not know the exact date on which fertilization occurred, and the date can be known accurately for women who have undergone ICSI; 

The fertilization date can be approximately calculated by adding 11-21 days to the date of the first day of the last menstrual cycle. 

For example, if the date of the last menstrual period is January 1, the fertilization date can be calculated as follows: 

• Possible fertilization date: January 12 - January 22. 
• The expected date of birth is during the period: October 3 - October 13. 

How can the age of the fetus be determined? 

Although pregnancy calculators are based on calculating the duration of pregnancy from the date of the first day of the last menstrual period, the actual development of the fetus does not begin until after fertilization occurs, that is, the time when the fetus began to grow and not from the beginning of pregnancy, so determining the age of the fetus accurately is done through an ultrasound examination (ultrasound). 

Through an ultrasound examination, the doctor measures the total length from head to toe, and through these measurements, the age of the fetus can be determined. 

Ultrasound can also be used to determine the likely birth site and is more accurate than calculating the date of the last menstrual period, and the doctor usually uses the result of both methods together to obtain accurate results. 

In conclusion, a pregnancy calculator can be used to obtain a possible date of birth. However, its results are not decisively accurate, and birth can occur within two weeks before or after the date specified by the calculator, so it is always preferable to follow up with a doctor so that the duration of pregnancy and the age of the fetus can be measured accurately.

Fetal growth and development chart 

Together we learn about the fetal growth and development chart that the pregnancy calculator usually shows with some pictures for each week including the length, weight, and new fetal skills:

Frequently asked questions 

We learn more about the most common questions we receive and answer them in detail:

Does the pregnancy calculator determine the sex of the fetus? 

No, the pregnancy calculator does not determine the sex of the fetus, and no program can determine that. It depends only on 3D imaging of the fetus, and some calculators may only give predictions about the sex of the fetus, but they are inaccurate.

What is the most important information provided by the pregnancy calculator?

The most important information provided by the pregnancy calculator is determining which week of pregnancy you are in now, the expected date of birth according to the date of the last menstrual period, along with information about the stages of development and growth of the fetus by week or hair, and some advice for pregnant women.

Sources and references

Pregnancy Due Date Calculator – americanpregnancy

Due date calculator – nhs