What is the patient's gestational age?

Determining Gestational Age

Gestational age is calculated as completed weeks and days from the first day of the last menstrual period, with a normal pregnancy lasting 40.0 weeks (280 days). 1

Standard Definition and Calculation

Gestational age is reported using the following format:

• Calculated from the first day of the last menstrual period (LMP)
• Reported as completed weeks and days
• Also known as menstrual age or clinical age 1

Methods for Determining Gestational Age

1. Last Menstrual Period (LMP)

• Traditional method but has limitations
• Assumes accurate recollection of dates and conception on day 14
• LMP-based estimates tend to be 0.8 days longer on average than ultrasound estimates 2
• Less reliable than ultrasound, with a coefficient of variation of 4.9% (compared to 3.7% when measured from ovulation) 3

2. Early Ultrasound (Gold Standard)

• Most accurate method, especially when performed before 24 weeks
• Estimates gestational age within 5-14 days 4
• First trimester ultrasound is generally considered the most accurate method 2
• Ultrasound performed at 16-18 weeks serves as a "gold standard" against which LMP estimates can be compared 5

3. In Vitro Fertilization/Ovulation Induction

• Provides accurate gestational age assignment when available 4
• In assisted reproductive techniques, gestational age is calculated by adding 14 days to completed weeks since fertilization 4

Accuracy Comparison

• Early ultrasound (14-24 weeks) predicts expected date of delivery within 14 days of actual delivery in 88.1% of cases 6
• LMP predicts expected date of delivery within 14 days in only 66.2% of cases 6
• LMP tends to misclassify more births as preterm (9.9%) or post-term (11.4%) compared to ultrasound (1.9% post-term) 6

Clinical Implications of Inaccurate Dating

Inaccurate dating can lead to:

• Unnecessary induction of labor
• Increased risk of cesarean section
• Increased maternal and neonatal morbidity 5
• Misclassification of pregnancies as preterm or post-term 6

Pregnancy Subcategories by Gestational Age

• First trimester: ≤13 weeks 6 days
• Term pregnancy: 37 weeks 0 days to 41 weeks 6 days
  • Early term: 37 weeks 0 days through 38 weeks 6 days
  • Full term: 39 weeks 0 days through 40 weeks 6 days
  • Late term: 41 weeks 0 days through 41 weeks 6 days
• Post-term: ≥42 weeks 0 days 1

Factors Affecting Gestational Length

• Maternal age (older mothers tend to have longer gestations) 3
• Maternal birth weight (heavier mothers at birth have longer gestations) 3
• Previous pregnancy history (an individual woman's deliveries tend to occur at similar gestational ages) 3
• Implantation timing (conceptions that took longer to implant also took longer from implantation to delivery) 3

Best Practice for Determining Gestational Age

1. Use early ultrasound (preferably first trimester) as the primary method
2. Consider LMP if early ultrasound is not available
3. If significant discrepancy exists between LMP and ultrasound dates (>7 days), rely on ultrasound dating
4. For pregnancies resulting from assisted reproductive technology, calculate from fertilization date plus 14 days

Remember that accurate dating is crucial for appropriate management of pregnancy, particularly for timing interventions like antenatal corticosteroids, planning delivery in high-risk pregnancies, and correctly classifying births as preterm or post-term.