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The Formula for Calculating Gestational Age, Really Easy!

November 17, 2022

The Formula for Calculating Gestational Age, Really Easy!

In general, gestational age is determined based on the date of LMP or last menstrual period. However, what if you and your partner forget the LMP date? Don’t worry because several formulas for calculating gestational age can be used, even if you forget the LMP date.

The pregnancy period until the delivery itself is approximately 38-40 weeks after the fertilization process. During this time, you and your partner should schedule several examinations with the obstetrician to see the fetus’s condition and its development from time to time.

To make it easier for mothers-to-be to calculate gestational age, we have summarized several methods used below. Good luck!

Ways to Calculate Gestational Age

There are many ways to find out the gestational age, either manually or with the help of modern tools. Here are some options that you can use to know your partner’s gestational age.

1. Calculate Gestational Age Manually

Finding out the actual gestational age can be calculated manually by paying attention to the last menstrual cycle. This method cannot be used to determine the exact date the baby will be born.

Here is the formula for calculating the gestational age manually:

As previously mentioned, to find out the gestational age, you need data on the first day of the last menstrual period or commonly abbreviated as LMP. Then the information can be processed using the Naegele formula as below.

  • LMP date + 7
  • Last menstrual month – 3
  • Last menstruation year + 1

Example of the first calculation scenario:

For example, if the mother-to-be’s LMP is on November 2nd , 2021, then the calculation is as follows:

  • 2 + 7 = 9
  • 11 – 3 = 9
  • 2021 + 1 = 2022

So that means the mother-to-be’s EDD will fall on September 9th, 2022.

Example of the second calculation scenario:

An example of a calculation with this formula can be used when the first day of the last menstrual period (LMP) for the mother-to-be falls between January – March or a month that cannot be reduced by 3. If such a condition occurs, the last menstrual month is added by 9, and the year is not added 1 or remains.

For example, the LMP of a mother-to-be falls on January 11th, 2022, so an example of the calculation formula looks like this:

  • 11 + 7
  • 1 + 9
  • 2022

Or EDD will be on October 18th, 2022.

How? The formula for calculating gestational age above is very simple and easy. Gestational age calculation can be done manually without the help of a midwife, which is indeed an advantage of the Naegele formula. So, if you are on a pregnancy program, be sure to record your menstrual cycle so that the data can be used to calculate gestational age.

2. Knowing Gestational Age with Ultrasound

Ultrasound examination has long been known to calculate gestational age with high accuracy.

The ultrasound examination procedure itself is divided into two types. The first is the transvaginal ultrasound, commonly done when the mother-to-be is still in the early stages of gestation. The second is a transabdominal ultrasound. This procedure is usually chosen when the fetus is already in a state of enlargement. The examination is carried out using sensors attached to the abdominal wall.

Interestingly, in addition to having an accuracy of up to 95% in calculating gestational age, Ultrasonography (USG) procedures can also be used to determine the sex of the unborn baby, check the development of the womb, and ensure the health of the fetus.

In normal pregnancy conditions, the obstetrician usually advises ultrasound examination when the gestational age has entered 4-5 weeks.

3. Using 4 ⅓ Formula

Besides the Naegele formula, there is one more formula that you can use to calculate the age of the fetus in the womb, namely the 4 ⅓ formula. This is an example of applying the formula.

Gestational age = {(Current date – LMP) x (4 ⅓)}

An example of a calculation scenario is as follows:

  • Today is July 14th, 2021 (2-11-2020)
  • Mother-to-be’s LMP falls on April 7th, 2021 (7-8-2020)

That means:

Gestational age = {(14 – 7), (7-4) x (4 ⅓) 

Days are subtracted from days, months are subtracted from months.

Gestational age = {(7), (3) x (4 ⅓) 

Multiplication is only done in months. Days don’t need to be multiplied by 4 ⅓

Gestational age = 7 days + 13 weeks = 1 week + 13 weeks = 13 weeks

So the gestational age in the data above is 14 weeks.

It is undeniable that the formula for calculating gestational age using the 4 ⅓ methods is a little more complex than the previous method. However, this method can also be used as a benchmark for obtaining EDD.

4. Using Pregnancy Calculator

In this era of technology development, smartphones are equipped with sophisticated applications. However, now there is also an application to calculate gestational age in the form of an online calculator that can facilitate the calculation of gestational age.

This method uses the same method as manual calculations, so you need data on the last menstruation period (LMP) and the menstrual cycle. You need to enter the date and month of the LMP and the menstrual cycle for how it works. Then the calculator will automatically provide the calculation results instantly.

Ways to Calculate Gestational Age if You Forget LMP

Sometimes because of busyness and many thoughts, expectant mothers do not remember when the last menstruation period (LMP) was. As a result, you and your partner have difficulty determining the gestational age manually. This condition is not a big problem because it can easily be overcome using the following method.

1. Using the Scan Tool

Based on data from the American Journal of Obstetrics and Gynecology, calculating the age of the fetus using a dating scan or ultrasound method is very common. Especially for couples who do not know LMP.

The calculation process itself is based on a series of measurement procedures in the gestational sac and the fetus or embryo itself.

2. Through Physical Examination

Although the results are less accurate and cannot be used as a benchmark, a physical examination can determine gestational age. The procedure and the examination process usually involve measuring the part of the pelvis or uterus that will increase during pregnancy.

3. Using Fetal Motion Detection

Compared with the calculation method using modern formulas or tools, this method is indeed inferior in terms of accuracy. This is because the main indicator used in this method is the fetus’s motion detection process.

The method of detecting fetal movement itself will generally be complicated for prospective mothers undergoing their first pregnancy. On the other hand, expectant mothers who have been pregnant before will find it easier to feel and recognize fetal movements inside the stomach.

Medically, the condition where the fetus has started to move on its own is a sign that the gestation period has entered the age of 18-20 weeks. While in the second pregnancy and beyond, movement can occur more quickly, namely when the age of 16-18 weeks.

Well, those are ways to calculate gestational age. If you are currently pregnant, you can manually try the formula for estimating gestational age before checking with a gynecologist. However, if the data related to LMP is incomplete, it is better to carry out an ultrasound examination immediately or through a scan. Hope it is useful.

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