Gestational Age Calculator

What Is Gestational Age?

Gestational age is the standard measure of how far along a pregnancy has progressed. It is counted from the first day of the last menstrual period (LMP), not from the date of conception. This means gestational age includes approximately two weeks before fertilization actually occurs. For related pregnancy tracking, use the Pregnancy Due Date Calculator.

Gestational age is expressed in weeks and days (e.g., "32 weeks and 4 days" or "32+4"). A full-term pregnancy is considered 40 weeks (280 days) from the LMP. Understanding your gestational age helps your healthcare provider schedule appropriate prenatal tests, monitor fetal growth, and estimate your due date.

How to Calculate Gestational Age

Method 1: Last Menstrual Period (LMP)

The simplest and most common method. Count the number of days from the first day of your last menstrual period to today. This is based on Naegele's rule, which assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. The estimated due date (EDD) is calculated as LMP + 280 days.

Method 2: Ultrasound Measurements

Ultrasound biometry uses fetal measurements to estimate gestational age. Different measurements are appropriate for different stages of pregnancy:

• CRL (Crown-Rump Length): Best used at 6-14 weeks, accuracy ±3-5 days. Uses the Robinson & Fleming formula.
• BPD (Biparietal Diameter): Best used at 13-40 weeks, accuracy ±7-10 days. Uses the Hadlock formula.
• FL (Femur Length): Best used at 14-40 weeks, accuracy ±7-14 days. Uses the Hadlock formula.
• HC (Head Circumference): Best used at 13-40 weeks, accuracy ±7-14 days. Uses the Hadlock formula.

Gestational Age vs. Fetal Age

These two terms are often confused but refer to different measurements. Gestational age is counted from the first day of the LMP and is the standard used in clinical practice. Fetal age (also called embryonic or conceptional age) is counted from the date of conception, which is approximately 2 weeks after the LMP. For example, at 10 weeks gestational age, the fetal age is approximately 8 weeks. Medical providers always refer to gestational age when discussing pregnancy milestones.

The Three Trimesters

• First Trimester (Weeks 1-13): All major organs begin forming, heartbeat starts, highest risk of miscarriage.
• Second Trimester (Weeks 14-27): Rapid growth, movements felt, anatomy scan, gender may be visible.
• Third Trimester (Weeks 28-40): Lungs mature, brain develops rapidly, baby gains weight, preparation for birth.

Frequently Asked Questions

How is gestational age calculated from LMP?

Gestational age from LMP is calculated by counting the number of days between the first day of your last menstrual period and today (or a reference date). This is based on Naegele's rule, which assumes a 28-day cycle with ovulation on day 14. The estimated due date is LMP plus 280 days (40 weeks).

What ultrasound measurements are used to estimate gestational age?

Four common ultrasound measurements are used: Crown-Rump Length (CRL) for the first trimester (most accurate, within ±3-5 days), Biparietal Diameter (BPD) for the second and third trimesters, Femur Length (FL) from 14 weeks onward, and Head Circumference (HC) from 13 weeks onward. CRL is the gold standard in early pregnancy.

Which method of calculating gestational age is most accurate?

First trimester ultrasound using Crown-Rump Length (CRL) is the most accurate method, with accuracy of ±3-5 days. LMP-based calculation assumes a regular 28-day cycle, which may not apply to all women. When there is a discrepancy of more than 5-7 days between LMP and first trimester ultrasound, clinicians typically use the ultrasound dating.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of the last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (also called embryonic age or conceptional age) is counted from the actual date of conception. Gestational age is always about 2 weeks more than fetal age. Medical professionals use gestational age as the standard.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1-13, during which major organs form. The second trimester covers weeks 14-27, when the baby grows rapidly and movements are felt. The third trimester runs from week 28 to birth (typically week 40), when the baby gains weight and organs mature for life outside the womb.

When is the due date?

The estimated due date (EDD) is 40 weeks (280 days) from the first day of the LMP. However, only about 4% of babies are born on their exact due date. Most births occur within a window of 37-42 weeks. A pregnancy is considered early term at 37-38 weeks, full term at 39-40 weeks, late term at 41 weeks, and post-term at 42 weeks and beyond. Check your conception timeline with the Pregnancy Conception Calculator or track weight with the Pregnancy Weight Gain Calculator.