What are the guidelines for calculating fetal gestational age in all trimesters of pregnancy?

Guidelines for Fetal Gestational Age Calculation Across All Trimesters of Pregnancy

Gestational age should be calculated from the first day of the last menstrual period (LMP) and/or early ultrasound measurements, with first trimester ultrasound (ideally between 7-14 weeks) providing the most accurate dating with a standard deviation of ±7 days. 1

First Trimester (≤13 weeks 6 days)

Primary Methods

• Last Menstrual Period (LMP): Initial estimation based on first day of LMP
• Crown-Rump Length (CRL): Gold standard between 7-14 weeks
  • Provides accuracy of ±7 days when CRL measures between 39-84 mm 1
  • Should be used to establish or confirm gestational age when available

Special Situations

• IVF/Assisted Reproduction: Calculate by adding 14 days to fertilization date 1
• Other Assisted Reproductive Technologies: Date based on clinical factors such as:
  • Time of intrauterine insemination
  • Embryo transfer date
  • Other clinical parameters 1

Second Trimester (14 weeks 0 days to 27 weeks 6 days)

Recommended Measurements

• Head Circumference (HC): Most accurate single parameter 2, 3
• Corrected Biparietal Diameter (BPD): Highly accurate in second trimester 2
• Femur Length (FL): Should be used in combination with HC for best accuracy 3

Best Prediction Model

• Combined HC and FL provides superior accuracy compared to single measurements 3
• When FL unavailable, HC alone is the next best option 3
• Expected accuracy: ±12-14 days at 26 weeks gestation 3

Third Trimester (≥28 weeks 0 days)

Recommended Measurements

• Head Circumference (HC) and Femur Length (FL): Best combined predictors 3
• Corrected Biparietal Diameter: Also accurate in third trimester 2
• Expected accuracy: >±14 days in the third trimester 3

Limitations

• Accuracy decreases progressively as pregnancy advances 2, 3
• Variation in fetal size increases in later pregnancy
• Growth restriction may affect accuracy of standard biometric measurements

Clinical Documentation and Reporting

• Report gestational age as number of weeks plus number of days (e.g., 12 weeks 3 days) 1
• For research and standardized reporting, gestational age at birth should be reported as:
  • Median and interquartile range
  • Mean and standard deviation to support future meta-analysis 4

Special Considerations

Multiple Gestations

• Requires establishing chorionicity and amnionicity
• More frequent ultrasound examinations than singleton pregnancies
• Twin-specific protocols include more frequent growth assessments 1

Growth Abnormalities

• Standard biometry less accurate in cases of intrauterine growth restriction
• Advanced machine learning approaches may improve accuracy in these cases 5

Pitfalls and Caveats

• Relying solely on LMP for women with irregular cycles leads to inaccurate dating 1
• Avoid using the term "normal first-trimester pregnancy" as abnormalities may be undetectable at early gestational age 1
• Adding abdominal circumference (AC) to HC and FL does not improve accuracy 3
• Accuracy of all predictors worsens progressively as pregnancy advances 2, 3

Algorithm for Gestational Age Determination

1. First trimester (≤13 weeks 6 days):

   • Use CRL between 7-14 weeks (most accurate method)
   • If IVF/ART: Use conception date + 14 days

2. Second trimester (14-27 weeks 6 days):

   • Use combination of HC and FL measurements
   • If only one measurement available, prioritize HC

3. Third trimester (≥28 weeks):

   • Use combination of HC and FL measurements
   • Recognize increased margin of error (>±14 days)
   • Consider clinical context and earlier dating if available

4. When no early ultrasound available:

   • Use best available biometric measurements
   • Document clearly that dating is based on late ultrasound
   • Consider increased margin of error in clinical decision-making