Gestational Age Calculator: Calculate Gestational Age Using Due Date

Determine your pregnancy progress by calculating the gestational age from your estimated due date.

Pregnancy Progress Timeline (40 Weeks)

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What is Gestational Age?

Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman’s last menstrual period (LMP) to the current date. A normal pregnancy ranges from 38 to 42 weeks. This calculator helps you calculate gestational age using due date, a method commonly used when the LMP is unknown or when a more accurate due date has been established by ultrasound.

This measurement is crucial for healthcare providers to track a baby’s growth, schedule tests and procedures, and predict the due date. Expectant parents also use it to follow their baby’s development week by week.

Gestational Age Formula and Explanation

While gestational age is traditionally counted from the Last Menstrual Period (LMP), it’s often more practical to calculate it based on an Estimated Due Date (EDD), especially after an ultrasound. A standard pregnancy is considered to be 280 days (or 40 weeks) long. Our calculator uses this principle to work backward from your EDD.

The formula is:

Gestational Age (in days) = 280 days - (Days between Evaluation Date and Due Date)

The result in days is then converted into weeks and days for a more common representation. For more on pregnancy timelines, see our guide on the Trimester Chart.

Table of variables used to calculate gestational age using due date.

Variable	Meaning	Unit	Typical Range
Estimated Due Date (EDD)	The predicted date of delivery.	Date	A future date, typically 40 weeks after LMP.
Evaluation Date	The date for which you want to calculate the gestational age.	Date	Usually today’s date.
Standard Gestation	The average length of a full-term human pregnancy.	Days	280 days (40 weeks)
Gestational Age	The resulting length of pregnancy on the evaluation date.	Weeks and Days	0w 0d to ~42w 0d

Practical Examples

Example 1: Early in Pregnancy

• Inputs:
  • Estimated Due Date: October 15, 2026
  • Evaluation Date: March 10, 2026
• Calculation:
  • Days remaining until due date: 219 days
  • Gestational Age in days: 280 – 219 = 61 days
• Results:
  • Primary Result: 8 weeks and 5 days
  • Trimester: 1st Trimester

Example 2: Mid-Pregnancy

• Inputs:
  • Estimated Due Date: July 20, 2026
  • Evaluation Date: April 5, 2026
• Calculation:
  • Days remaining until due date: 106 days
  • Gestational Age in days: 280 – 106 = 174 days
• Results:
  • Primary Result: 24 weeks and 6 days
  • Trimester: 2nd Trimester

How to Use This Gestational Age Calculator

Using our tool to calculate gestational age using due date is simple. Follow these steps:

1. Enter the Estimated Due Date (EDD): Use the date picker to select the due date given to you by your doctor. This is the most important piece of information.
2. Select the Evaluation Date: This field defaults to today’s date. You can change it if you wish to find the gestational age on a different day in the past or future.
3. Review Your Results: The calculator will instantly update, showing the gestational age in weeks and days, the current trimester, total days pregnant, and days remaining until your due date.
4. Analyze the Chart: The visual chart shows your progress through the three trimesters, helping you contextualize how far along you are.

For those early in their journey, a Pregnancy Due Date Calculator can help establish this initial date.

Key Factors That Affect Gestational Age Calculation

Several factors can influence the accuracy of an EDD and, consequently, the gestational age calculation:

• Accuracy of Last Menstrual Period (LMP): The initial EDD is often based on the LMP. If your memory of the date is off or your cycles are irregular, the EDD may be adjusted.
• Ultrasound Dating: An early ultrasound (especially in the first trimester) provides the most accurate dating of a pregnancy by measuring the fetus. An EDD from an ultrasound is generally preferred over an LMP-based date.
• IVF Transfer Date: For pregnancies conceived via In Vitro Fertilization, the date of embryo transfer provides a very precise starting point, leading to a highly accurate EDD and gestational age.
• Cycle Length: The standard 280-day calculation assumes a 28-day menstrual cycle with ovulation on day 14. Women with longer or shorter cycles may have their EDD adjusted. Our Ovulation Calculator can help visualize cycle timing.
• Conception Date: If the exact date of conception is known, it provides a precise fetal age, but gestational age still adds two weeks to this to align with the standard LMP-based counting method.
• Fundal Height: In the second and third trimesters, your doctor may measure the fundal height (the distance from the pubic bone to the top of the uterus) to check if fetal growth is on track with the gestational age.

Frequently Asked Questions (FAQ)

1. Why is pregnancy counted as 40 weeks and not 9 months?

Forty weeks is actually slightly more than nine months. Because months have a variable number of days, counting in weeks provides a more accurate and consistent standard for tracking developmental milestones.

2. What if I go past my due date?

It’s very common to deliver before or after your EDD. A pregnancy is considered “term” between 37 and 42 weeks. This calculator will simply show a gestational age greater than 40 weeks if the evaluation date is past the due date.

3. How accurate is this calculator?

The calculation itself is precise. Its accuracy in reflecting your actual pregnancy timeline depends entirely on the accuracy of the Estimated Due Date you provide.

4. What are the trimesters?

Pregnancy is divided into three trimesters. The first is from week 1-13, the second from week 14-27, and the third from week 28 until birth. Each has distinct developmental phases for the baby.

5. Can I use my conception date instead?

This calculator is designed to calculate gestational age using due date. If you know your conception date, you can first use a Due Date Calculator to find your EDD, and then use that date here.

6. Is gestational age the same as fetal age?

No. Fetal age (or conceptual age) is the age of the fetus from the date of conception. Gestational age is typically two weeks longer because it is counted from the start of the last menstrual period, before conception occurred.

7. Where does the 280-day number come from?

It is the average length of a human pregnancy, equivalent to 40 weeks. It is calculated by adding 280 days to the first day of the last menstrual period (LMP).

8. Why does my doctor’s calculation differ slightly?

A doctor might adjust the EDD based on multiple factors, including different ultrasound measurements or a more detailed cycle history. Always defer to the dates provided by your healthcare professional.