## When you know you’re pregnant, you’ll certainly want to learn how to calculate gestational age to assess your baby’s development. AVAKids will guide you through some simple ways to calculate gestational age in the article below.
### 1. Purpose of Calculating Gestational Age
A pregnant woman’s gestation period is divided into three trimesters, each representing a specific stage of development for both mother and baby.
* **First Trimester:** Weeks 1-13, calculated from the time of fertilization until the 3rd month of pregnancy.
* **Second Trimester:** Weeks 14-27, calculated from the 3rd month to the 6th month of pregnancy.
* **Third Trimester:** Weeks 28-42, calculated from the 6th month to the 9th month of pregnancy.
[Related: Signs of a healthy developing fetus in the first 3 months that mothers can monitor themselves]
[Image: Calculating gestational age helps monitor fetal development in the uterus. Photo: Canva]
Calculating gestational age helps expectant mothers and doctors **monitor and evaluate the development of the fetus** in the uterus. It also helps doctors make decisions about necessary tests at each stage of pregnancy.
Additionally, calculating gestational age helps expectant mothers **predict their due date**, compare fetal growth and development across different stages of pregnancy, and make adjustments to their diet or care accordingly.
[Related: What is the size of a 5-month pregnant belly? What to do if the 5-month belly is still small?]
### 2. Top 6 Most Accurate Ways to Calculate Gestational Age
Some relatively simple and accurate ways to calculate gestational age that expectant mothers can apply include:
#### Calculating Gestational Age Based on Menstruation
Calculating gestational age based on the last menstrual period (LMP) is a common method. The **age of the fetus is calculated from the first day of the last menstrual period**, but there is typically a deviation of up to 2-3 weeks.
This method should only be used for mothers with relatively regular menstrual cycles. If the menstrual cycle is irregular or the mother doesn’t remember her LMP, this method shouldn’t be used.
[Image: Calculating gestational age based on menstruation. Photo: Canva]
#### Calculating Gestational Age Based on Ovulation Date
If the expectant mother remembers the date of intercourse, she can calculate gestational age based on the ovulation date. This method is quite accurate. Sperm can survive in a woman’s body for 5 days, but an egg can only survive for 24 hours after ovulation. Therefore, fertilization and embryo formation occur within this timeframe.
Calculation: If intercourse occurred on the ovulation date, which is 10/5/2022, the gestational age starts from 10/5 or 10/6.
This method should only be used for mothers with regular menstrual cycles, who remember the date of intercourse and know their ovulation date.
[Related: 6-Month Pregnant Belly Size and Things Mothers Should Note]
#### Calculating Gestational Age by Ultrasound
With ultrasound, the expectant mother doesn’t need to provide the date of her last menstrual period or ovulation date. Doctors will **calculate gestational age based on measurements of the gestational sac and the fetus**. In the first trimester, doctors will perform an **ultrasound and measure the crown-rump length (CRL)** of the fetus to calculate gestational age and determine the due date. The deviation in the due date is quite low, only around 3 days.
Expectant mothers should calculate the due date during this ultrasound because as the fetus grows, the deviation in the due date increases. If the due date is calculated in the second trimester, the deviation can be 1-2 weeks.
[Related: Fetal Femur Length Standards Mothers Need to Know]
#### Measuring Uterine Height
Calculating gestational age by measuring uterine height is a modern method. In this method, the doctor measures the **distance between the pubic bone and the fundus of the uterus**. In most pregnant women, when the **fetus is about 1 month old, the uterine height increases by about 4cm/month**. Therefore, the formula for calculating gestational age is as follows:
* If calculated by month: Gestational age = Uterine height/4 + 1.
* If calculated by week: Gestational age = Uterine height + 4.
Estimated gestational age based on uterine height:
* 12 weeks: Uterine height = 1/3 of the mother’s pubic bone.
* 16 weeks: Uterine height = 2/3 of the mother’s pubic bone.
* 20 weeks: Uterine height = level with the mother’s navel.
* 24 weeks: Uterine height is slightly above the mother’s navel.
* 28 weeks: Uterine height = 1/4 of the abdominal apex, above the mother’s navel.
* 32 weeks: Uterine height = 1/2 of the abdominal apex, above the mother’s navel.
* 36 weeks: Uterine height = 4/4 of the abdominal apex, above the mother’s navel.
#### Calculating IVF Gestational Age
This method is used for **in vitro fertilization (IVF)** pregnancies. In this method, parents and doctors know the exact date of fertilization, making it easier to calculate gestational age and the due date.
[Image: IVF gestational age calculation is used for in vitro fertilization cases. Photo: Canva]
The due date is calculated by adding 38 weeks from the date of fertilization (usually the 266th day of pregnancy).
It can also be calculated by adding 38 weeks and then subtracting the number of days the embryo was implanted. For example, if the embryo was implanted 2 days prior, subtract 2 days, meaning the due date would be on the 264th day of pregnancy.
#### Using Gestational Age Calculator Apps
With the development of the internet, many gestational age calculator apps are available. These methods can be performed online or through apps, making it very convenient for expectant mothers. However, the limitation of these apps is that the expectant mother must have a regular menstrual cycle and remember the correct cycle.
Some apps include:
* **What to Expect’s Pregnancy & Baby:** Tracks fetal development weekly, counts down to the due date.
* **Sprout:** Tracks fetal development with 3D images. The app also calculates gestational age, fetal weight, and predicts the due date.
* **MarryBaby:** Helps expectant mothers estimate their due date and calculate gestational age based on the time of intercourse.
[Related: What is Breech Presentation? Answers to Related Questions]
### 3. Should Gestational Age be Calculated by Week or Month?
Here’s a table converting gestational age from weeks to months:
| | Month | Weeks |
|—————-|——-|——-|
| First Trimester | 1 | 1-4 |
| First Trimester | 2 | 5-8 |
| First Trimester | 3 | 9-13 |
| Second Trimester| 4 | 14-17 |
| Second Trimester| 5 | 18-21 |
| Second Trimester| 6 | 22-26 |
| Third Trimester | 7 | 27-30 |
| Third Trimester | 8 | 31-35 |
| Third Trimester | 9 | 36-40 |
[Related: What is Fetal Presentation? What to Do with Abnormal Presentation?]
### 4. How to Calculate the Due Date Accurately
Expectant mothers can calculate their due date as follows:
* Due Date = Date of LMP + 7 days
* Due Month = Month of LMP (from April to December) – 3 months
* Or: Due Month = Month of LMP (from January to March) + 9 months
Example: Your LMP was 10/5/2022.
→ Your due date would be 7/12/2023.
The **accuracy of the due date is only 5%**, so it’s only an estimate. Typically, babies are born between 37-42 weeks of gestation, with a deviation of about 1 week from the calculated due date.
[Related: Fetal Heart Rate and Things Pregnant Mothers Need to Note]
### 5. Some Frequently Asked Questions About Calculating Gestational Age
#### How Many Days are in a Week of Pregnancy?
Typically, pregnancy lasts 40 weeks, equivalent to 280 days, from fertilization until birth.
The first day of pregnancy is determined as the first day of the LMP. Then, the following weeks are added and calculated normally. This is a simple and accurate way to calculate gestational age. Remembering the first day of the LMP allows for calculation.
[Related: Biparietal Diameter Standards by Gestational Week and Foods to Improve Size]
#### Why is the Gestational Age 4 Weeks When I’m 5 Weeks Pregnant?
Generally, when a mother is 1 week pregnant and the fetus is entering the 2nd week of gestation, the mother is still considered 1 week pregnant.
Specifically:
* During the first week of gestation, the mother is considered not yet pregnant.
* In the second week, the fetus is 2 weeks old, but the mother is considered 1 week pregnant.
* Similarly, when the mother is 5 weeks pregnant, it’s normal for the doctor to say the fetus is 4 weeks old.
#### How Many Weeks is Considered Full Term?
A full-term pregnancy is 40 weeks from the first day of the LMP. In reality, a fetus is viable outside the womb from 38 weeks onwards.
Babies born between 39 and 40 weeks have the fewest complications. Babies born earlier or later than this time have a higher risk of complications.
* Before 37 weeks: Preterm birth.
* 37-38 weeks: Early term birth.
* 39-40 weeks: Full term birth.
* 41 weeks: Late term birth.
* 42 weeks or later: Postterm birth.
However, each pregnant woman’s physical condition is different, so the delivery time also varies. Especially for first-time mothers, the actual delivery time is often 7-10 days earlier than the calculated due date.
[See also:
* Fetal Foot Length Standards and Influencing Factors
* Fetal Development at 12 Weeks and Necessary Tests
* Fetal Development at 30 Weeks and Notes for a Smooth Delivery]
_Above are some ways to calculate fetal age that mothers can use to predict their due date. However, these are general calculations. In reality, the time of delivery depends on various factors, so gestational age and due date calculations are for reference only. It’s best to consult with your doctor during regular checkups._
**Compiled by Nguyet Minh**
**Reviewed by Ngoc Thanh**