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Understanding Gestational Age and How It’s Calculated

Gestational age describes how far along a pregnancy is, expressed in completed weeks and days. It is fundamental for monitoring fetal growth, scheduling prenatal testing, and estimating delivery timing. The most common way to calculate it is from the first day of the last menstrual period (LMP), even though conception occurs roughly two weeks later. This convention provides consistency in medical practice and allows gestational age to be calculated even if conception date is unknown.

1. How gestational age is calculated

By convention, pregnancy duration is about 280 days (40 weeks) from the first day of LMP. Gestational age (GA) is calculated by counting the number of days or weeks that have passed since that date.

Formula:

Gestational Age = Current Date − LMP

Alternatively, if the due date (EDD) is known, gestational age can be computed as:

Gestational Age = 40 weeks − (EDD − Current Date)

2. Using ultrasound for gestational dating

An early ultrasound (typically between 7–13 weeks) provides the most accurate gestational age estimation, based on the measurement of the crown–rump length (CRL). If ultrasound dating differs from LMP by more than 7 days, the ultrasound-based gestational age is usually used for clinical purposes.

3. Trimesters and key milestones

• First trimester: 0–13 weeks 6 days — organ development and early screening tests.
• Second trimester: 14–27 weeks 6 days — anatomy scan and fetal movement.
• Third trimester: 28 weeks until birth — growth monitoring and labor preparation.

4. Importance of gestational age

• Determines when prenatal tests (e.g., NT scan, glucose screening) should occur.
• Guides interpretation of fetal growth measurements.
• Determines viability and preterm vs. term birth classification.
• Supports accurate timing for induction or cesarean planning.

5. Examples

Example 1: LMP = January 1 → Current date = March 15 → Gestational age ≈ 10 weeks 2 days. Due date = October 8.

Example 2: Due date = August 1 → Current date = May 1 → Gestational age ≈ 27 weeks 0 days.

6. Accuracy and limitations

LMP-based dating assumes a 28-day cycle with ovulation on day 14. If cycles are longer or shorter, gestational age estimates may vary. Ultrasound dating early in pregnancy is the most reliable method for precise dating.

7. Clinical significance

• Preterm: < 37 weeks
• Term: 37–42 weeks
• Post-term: > 42 weeks

Gestational age informs the classification of preterm births and helps manage high-risk pregnancies.

8. Factors affecting estimation

Uncertain LMP, irregular cycles, or contraceptive use before conception can make LMP-based estimates less reliable. Combining clinical, ultrasound, and historical information provides the most accurate picture.

9. Tools like this calculator

Online gestational age calculators provide a quick reference for expectant parents and healthcare professionals. However, the results should always be confirmed by a healthcare provider, especially for medical decisions.

Summary

The Gestational Age Calculator estimates how many weeks pregnant you are and your likely due date based on LMP or EDD. Understanding gestational age helps ensure appropriate prenatal testing, safe delivery timing, and overall pregnancy tracking. While simple to use, this calculator is a guide and should be used alongside professional prenatal care.

Disclaimer: This tool is for educational use only. Always confirm your gestational age with your healthcare provider.