Gestational Age Calculator
Calculate precise gestational age in weeks and days from the first day of your last menstrual period (LMP). Shows developmental milestones and expected growth markers.
This free online gestational age calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
Days since the first day of your last period
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Gestational Age Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Gestational Age Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Gestational Age Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Gestational Age Calculator for personal health tracking and wellness monitoring, establishing a baseline and tracking changes over time.
- •Use it when recording fitness metrics to track progress toward health or athletic goals.
- •Use it to compare measurements before and after a lifestyle, diet, or training change to quantify the impact.
- •Use it as a conversation starter before a doctor's appointment, bringing objective data to discuss with a healthcare professional.
About This Calculator
The Gestational Age Calculator is a free health and wellness calculation tool designed for personal use and general informational purposes. Calculate precise gestational age in weeks and days from the first day of your last menstrual period (LMP). Shows developmental milestones and expected growth markers. This calculator provides reference values based on established health screening formulas and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Results may vary based on individual factors not captured by the calculation. Consult a qualified healthcare provider for personalized health guidance. All calculations are performed locally in your browser — no personal health data is transmitted or stored.
About Gestational Age Calculator
The Gestational Age Calculator computes how far along a pregnancy has progressed in weeks and days, counted from the first day of the last menstrual period (LMP). Gestational age is the universal clinical standard for tracking pregnancy progress, scheduling prenatal tests, assessing fetal development, and determining viability. It is used for all medical decisions, from timing the anatomy scan (18-22 weeks) to determining term status (37-42 weeks). This calculator also shows trimester classification and percentage of pregnancy completed.
The Math Behind It
Formula Reference
Gestational Age
GA = Days since LMP / 7 (weeks + days)
Variables: LMP = first day of last menstrual period
Worked Examples
Example 1: First Trimester
LMP was 84 days ago
12 weeks 0 days, 1st trimester, 30% of pregnancy completed.
Example 2: Third Trimester
LMP was 245 days ago
35 weeks 0 days, 3rd trimester, 87.5% complete.
Common Mistakes & Tips
- !Using the conception date instead of the LMP. Gestational age always counts from the first day of the last menstrual period.
- !Not accounting for irregular cycles. In cycles longer than 28 days, LMP-based dating overestimates gestational age because ovulation occurred later than expected.
- !Confusing weeks and months. Pregnancy months are not exactly 4 weeks; 40 weeks = approximately 9 months and 1 week.
Related Concepts
Used in These Calculators
Calculators that build on or apply the concepts from this page:
Frequently Asked Questions
How is gestational age different from fetal age?
Gestational age counts from the LMP and is approximately 2 weeks longer than fetal age (which counts from conception). A pregnancy at 12 weeks gestational age has a fetal age of about 10 weeks.
What if my ultrasound date differs from my LMP date?
First-trimester ultrasound is more accurate than LMP dating. If the ultrasound estimate differs by more than 5-7 days from LMP, your provider will typically adjust your dates to match the ultrasound.
When does each trimester start?
First trimester: weeks 1-13 (conception through early organogenesis). Second trimester: weeks 14-27 (rapid growth, anatomy scan). Third trimester: weeks 28-40 (maturation, preparation for birth).