Gestational Age Calculation Methods
Gestational age is most accurately calculated using ultrasound measurements, particularly crown-rump length in early pregnancy, which provides better accuracy than last menstrual period dating. 1
Primary Methods for Calculating Gestational Age
Last Menstrual Period (LMP) Method
- Gestational age is defined as the number of days elapsed since the first day of the last normal menstrual period 2
- This is the most common traditional method for determining gestational age 2
- LMP dating is sufficiently accurate for many screening purposes, but less precise than ultrasound dating 2
- LMP tends to slightly overestimate gestational age compared to ultrasound (median 276.0 days vs. 273.9 days in one study) 3
- When clinical information shows LMP dating to be in error, adjustments should be made 2
Ultrasound Dating Methods
- Ultrasound measurement is more accurate than LMP and improves both sensitivity and specificity of screening 2, 1
- Crown-rump length (CRL) in early pregnancy (11-13 weeks) provides the most accurate estimate of gestational age 1, 4
- In the second trimester, composite ultrasound dating based on multiple measurements is accurate to within 10 days 2
- For pregnancies at 14 weeks or later, biparietal diameter (BPD) measurement is recommended, especially when LMP is uncertain 2
- Head circumference (HC) combined with femur length (FL) provides the best gestational age prediction in late pregnancy 4
Expressing Gestational Age
- Gestational age should be expressed in completed weeks (e.g., 15 weeks and 5 days is 15 completed weeks) 2
- For improved screening performance, express as weeks and days or decimal weeks (e.g., 15 weeks and 5 days is 15.7 weeks) 2
- Rounding to the nearest week (e.g., 15 weeks and 5 days to 16 weeks) is not recommended 2
Special Considerations
Dating Discrepancies
- When discrepancies exist between LMP and ultrasound dating, ultrasound dating should be preferred, especially CRL measurements from the first trimester 1
- Document both LMP-based and ultrasound-based dates, but use ultrasound-based dates for clinical decision-making 1
- Agreement between LMP and ultrasound estimates is typically within ±14 days for about 88% of women 3
Factors Affecting Gestational Age Calculation
- Menstrual cycle length affects gestational age calculation when using LMP method - women with longer cycles tend to have more births classified as post-term 5
- Maternal characteristics such as young age, race/ethnicity, and non-optimal body weight can increase discrepancies between LMP and ultrasound dating 6
- When conception date is known, gestational age can be calculated as the number of days from conception plus 14 days 2
Clinical Applications
- Accurate gestational age determination is crucial for appropriate timing of screening tests, such as those for Down syndrome and neural tube defects 1
- Precise dating is essential for correct interpretation of serum screening results, which are highly dependent on gestational age 1
- Maternal serum AFP levels increase by approximately 10-15% per week in the second trimester, making accurate gestational age determination critical for interpretation 2