What is the classification and management of gestational age in pregnant individuals?

Gestational Age Classification

Gestational age (GA) is reported as the number of weeks and days of pregnancy duration, with the first trimester defined as GA ≤13 weeks 6 days, and accurate dating is best established using first-trimester crown-rump length (CRL) measurement between 8-13 weeks, which provides accuracy within ±5-7 days. 1, 2

Standard Classification Framework

Trimester Definitions

• First trimester: GA ≤13 weeks 6 days 1
• Second trimester: 14 weeks 0 days through 27 weeks 6 days (standard obstetric practice)
• Third trimester: ≥28 weeks 0 days (standard obstetric practice)

Reporting Format

• Express GA as weeks and days (e.g., 15 weeks 5 days) or decimal weeks (15.7 weeks) rather than rounded weeks 1, 2
• Completed weeks format (15 weeks and 5 days = 15 completed weeks) is acceptable but less precise 1
• Never round to the nearest week as this significantly reduces screening accuracy 1, 2

Optimal Dating Hierarchy for Accurate GA Assignment

First-Line Method: First Trimester CRL

• CRL measurement between 8-13 weeks is the gold standard, providing accuracy within ±5-7 days 2
• Can be as precise as ±1.5 days compared to true gestational age 2, 3
• When discrepancy ≥5 days exists between LMP and CRL dating, adopt the CRL-based GA for all clinical decisions 2, 4
• Record both LMP-based and ultrasound-based dates in the medical record, but use ultrasound-based date for clinical decisions 2

Second-Line Method: Second Trimester Composite Ultrasound

• Composite ultrasound dating using multiple fetal measurements between 18-22 weeks is accurate to within 10 days 2
• Use when first trimester dating was missed 2
• Multiple measurements (biparietal diameter, head circumference, abdominal circumference, femur length) are more accurate than single measurements 1

Third-Line Method: BPD Alone

• Biparietal diameter (BPD) measurement alone at ≥14 weeks when LMP is uncertain or discrepant 1, 2
• Specifically recommended because it rules out anencephaly and improves detection of open spina bifida 1, 2

Special Population: ART Pregnancies

• Calculate GA by adding 14 days to the number of completed weeks since fertilization 2
• This provides the most accurate dating possible since exact fertilization date is known 2, 3

Clinical Context-Specific Classifications

Fetal Growth Restriction (FGR) Classification

• Early-onset FGR: diagnosed before 32 weeks gestation 1
• Late-onset FGR: diagnosed at or after 32 weeks gestation 1
• The 32-week cutoff maximizes differences in associated comorbidities and pregnancy outcomes 1
• Early-onset FGR is typically more severe, follows established Doppler deterioration patterns, and is more commonly associated with maternal hypertensive disorders 1
• Late-onset FGR represents 70-80% of FGR cases and is typically milder 1

Severity Classification for FGR

• Severe FGR: estimated fetal weight (EFW) <3rd percentile 1
• Moderate FGR: EFW 3rd-10th percentile 1
• EFW <3rd percentile increases stillbirth risk approximately 3-fold over 3rd-5th percentile at nearly all gestational ages 1

Critical Management Implications by GA

Corticosteroid Administration

• Give prenatal corticosteroids between 24+0 and 34+0 weeks gestation 1
• May be given up until 38+0 weeks for elective cesarean section 1
• Multiple steroid courses are not recommended 1

Magnesium Sulfate for Neuroprotection

• Administer MgSO4 for fetal neuroprotection before 32 weeks gestation 1

Delivery Timing in FGR with Abnormal Doppler

• Absent end-diastolic flow: consider delivery no later than 34 weeks 1
• Reversed end-diastolic flow: consider delivery no later than 30 weeks 1
• Earlier delivery may be indicated with poor interval growth or deteriorating sonographic variables 1

Common Pitfalls to Avoid

Dating Errors

• Never average LMP and ultrasound dates - always use the ultrasound date when available in the first trimester 2
• LMP-based dating shows systematic bias, with 17.2% of records having absolute differences >14 days compared to ultrasound 5
• LMP estimates systematically overstate gestation duration by approximately 2.8 days on average, likely due to delayed ovulation 6
• LMP-based preterm birth estimates have only 64.3% sensitivity and 58.7% positive predictive value 5

Measurement Technique

• Ensure proper CRL measurement: measure greatest length from crown to rump, excluding limbs and yolk sac 2
• Early ultrasound systematically underestimates GA of smaller fetuses by 1-2 days, but this is small compared to LMP error 5

Screening Test Interpretation

• Do not reclassify screen-positive results to screen-negative based on dating changes unless revised GA differs by ≥10 days 2
• Maternal serum AFP levels increase 10-15% per week in second trimester, making accurate dating essential 1, 2, 4
• Never use package insert medians for AFP screening without establishing laboratory-specific normative data 1, 2

Irregular Menstrual Cycles

• For irregular cycles, adjust Naegele's rule by adding the difference between the patient's cycle length and standard 28-day cycle 4
• For a 35-day cycle, add 7 days to account for delayed ovulation occurring around day 21 instead of day 14 4