How is the week of gestation (gestational age) calculated?

Calculation of Gestational Age (Week of Gestation)

Gestational age (GA) is calculated from the first day of the last menstrual period (LMP) and/or early ultrasound measurements, and is reported as number of weeks plus number of days. 1, 2

Standard Methods for Calculating Gestational Age

LMP-Based Dating

• Gestational age begins counting from the first day of the last menstrual period
• This method assumes a standard 28-day menstrual cycle with ovulation at mid-cycle
• LMP-based estimates are typically 0.8 days longer on average than ultrasound estimates 3
• Limitations include:
  • Uncertainty about LMP date
  • Irregular menstrual cycles
  • Bleeding not associated with menses
  • Delayed ovulation

Ultrasound-Based Dating

• Crown-rump length (CRL) measurement between 7-14 weeks provides accuracy of ±7 days 2
• First trimester ultrasound is generally considered more accurate than LMP alone 3
• For dating after the first trimester:
  • Multiple parameters (head circumference, femur length) provide accuracy of ±10-14 days at 20-26 weeks 2
  • Accuracy decreases as pregnancy progresses

Special Circumstances

Assisted Reproduction

• For IVF pregnancies, gestational age is calculated by adding 14 days to the fertilization date 2
• In other assisted reproductive technologies, pregnancy is dated based on clinical factors such as:
  • Time of intrauterine insemination
  • Embryo transfer date
  • Other clinical parameters 1

Clinical Implications of Gestational Age Calculation

Importance of Accurate Dating

• Critical for proper timing of:
  • Prenatal screening tests
  • Assessment of fetal growth
  • Diagnosis of preterm or post-term pregnancies
  • Timing of interventions like induction of labor 2
  • Gestational diabetes screening (typically at 24-28 weeks) 2

Discrepancies Between Methods

• When LMP and ultrasound dates differ significantly, ultrasound dating is generally preferred, especially when performed in early pregnancy
• Discrepancies between LMP and ultrasound dating are more common among:
  • Young women
  • Non-Hispanic Black and Hispanic women
  • Women of non-optimal body weight
  • Mothers of low-birthweight infants 3

Best Practice Recommendations

• First trimester ultrasound (ideally between 7-14 weeks) provides the most accurate dating
• When ultrasound is unavailable, LMP-based dating with careful history taking is a reasonable alternative 4
• For clinical documentation, gestational age should be reported as number of weeks and number of days (e.g., 12 weeks 3 days) 1
• The first trimester is defined as GA ≤ 13 weeks 6 days 1

Common Pitfalls to Avoid

• Relying solely on LMP for women with irregular cycles can lead to inaccurate dating
• Using the term "normal first-trimester pregnancy" should be avoided since abnormalities may be undetectable at early gestational age 1
• Failing to adjust for cycle length can result in misclassification of post-term pregnancies 5
• Neonatal estimates of gestational age (like Ballard or Dubowitz scoring) are the least precise dating method and should be used only when other methods are unavailable 6