Gestational Age Calculator
Estimate gestational age and trimester based on a reference date using common pregnancy calculation methods.
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Result
Enter values above and click Calculate to see your result.
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Formula
The calculator derives an LMP date from the chosen method and computes weeks and days since that reference.
Worked Example
How Gestational Age is Calculated
- A full-term pregnancy is 40 weeks (280 days) from the LMP, divided into three trimesters
- First trimester: weeks 1 to 12. Second trimester: weeks 13 to 26. Third trimester: weeks 27 to 40
- Gestational age from LMP is typically about 2 weeks more than fetal age (time since actual conception)
- Ultrasound dating may adjust gestational age if the measured fetal size differs significantly from LMP dating
This calculator provides an estimate based on your reference date. Your healthcare provider may adjust dating based on ultrasound measurements and clinical assessment.
You can also calculate changes using our Corrected Age Calculator, Exact Age Calculator or Birthday Countdown Calculator.
Frequently Asked Questions
Which calculation method is used by default?
The calculator uses the last menstrual period (LMP) method by default, counting from the first day of your last period. This is the standard method used in obstetric dating.
Is this exact for every pregnancy?
No. Pregnancy dating can vary due to irregular cycles, uncertain LMP dates, and individual variation. Ultrasound dating in the first trimester is often more accurate. This is a simplified planning estimate.
Why does gestational age start before conception?
By convention, gestational age counts from the first day of the last menstrual period, which is approximately 2 weeks before ovulation and conception. This is because LMP is a known date while conception date is usually uncertain.
What is the difference between gestational age and fetal age?
Gestational age counts from the LMP (about 40 weeks for a full pregnancy). Fetal age counts from conception (about 38 weeks). Gestational age is always approximately 2 weeks more than fetal age.
When should I see a doctor instead of using this calculator?
This calculator is for informational planning only. Schedule prenatal care with a healthcare provider as soon as you know you are pregnant. They will confirm dating with clinical assessment and ultrasound.
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