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.

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
• Ovulation occurring on day 14
• Full-term pregnancy = 280 days (40 weeks)

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:

Gestational Age vs. Fetal Age

These two terms are often confused but refer to different measurements:

• Gestational age: Counted from the first day of the LMP. This is the standard used in clinical practice.
• Fetal age (embryonic/conceptional age): 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

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: 37 weeks 0 days – 38 weeks 6 days
• Full term: 39 weeks 0 days – 40 weeks 6 days
• Late term: 41 weeks 0 days – 41 weeks 6 days
• Post-term: 42 weeks 0 days and beyond

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 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.

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.