How is the age of gestation determined in the context of an abortion?

Determining Gestational Age in the Context of Abortion

Gestational age is calculated by the best obstetric estimate using early ultrasound crown-rump length (CRL) measurement as the gold standard, supplemented by last menstrual period (LMP) dating when ultrasound is unavailable. 1

Primary Method: First-Trimester Ultrasound

The most accurate method for determining gestational age is first-trimester ultrasound using crown-rump length (CRL) measurement between 8-13 weeks, which provides accuracy within ±5-7 days. 2

• When CRL measurement is available and shows a discrepancy of ≥5 days from LMP dating, the ultrasound-based gestational age should be adopted for all clinical decision-making 2
• CRL measurement provides the highest precision, with accuracy as precise as ±1.5 days compared to true gestational age 2
• The CRL should be measured as the greatest length from crown to rump, excluding limbs and yolk sac 2

Alternative Dating Methods

Last Menstrual Period (LMP)

• LMP remains the most common method for initial gestational age assessment, though it is less accurate than ultrasound 1
• This method assumes regular menstrual cycles and accurate recall of the first day of the last menstrual period 1

Second-Trimester Ultrasound

• When first-trimester dating is unavailable, composite ultrasound dating using multiple fetal measurements at 18-22 weeks provides accuracy within ±10 days 2
• Biparietal diameter (BPD) measurement at ≥14 weeks can be used when LMP is uncertain, with the added benefit of ruling out anencephaly 1, 2

Assisted Reproductive Technology (ART)

• For pregnancies conceived through ART, gestational age is calculated by adding 14 days to the number of completed weeks since fertilization 1
• This method provides highly accurate dating since the exact fertilization date is known 2

Clinical Importance in Abortion Context

Accurate gestational age determination is critical because maternal morbidity and mortality from induced abortion increase significantly with advancing gestation. 1

Mortality Risk by Gestational Age

• At 11-12 weeks' gestation: 1.1 deaths per 100,000 abortions 1
• At 16-20 weeks' gestation: 6.9 deaths per 100,000 abortions 1

Complication Rates by Gestational Age

• Major complication rate at 11-12 weeks: 0.8% 1
• Major complication rate at 17-20 weeks: 2.2% 1
• Major complications include prolonged fever, hemorrhage requiring transfusion, and pelvic organ injury 1

Medical Abortion Efficacy Considerations

Medical abortion efficacy decreases with advancing gestational age, making accurate dating essential for appropriate method selection. 3, 4

• At 9-10 weeks: surgical intervention needed in 0.9% of cases 3
• At 10-11 weeks: surgical intervention needed in 5.3% of cases 3
• At 11-12 weeks: surgical intervention needed in 6.2% of cases 3
• At 12-13 weeks: surgical intervention needed in 7.9% of cases 3

Medical abortion can be safely performed as early as pregnancy is diagnosed (before 42 days/6 weeks), with efficacy rates of 98% that are comparable to later first-trimester procedures 5

Documentation Requirements

Both LMP-based and ultrasound-based dates should be recorded in the medical record, but the ultrasound-based date must be used for clinical decisions when available. 2

• Express gestational age as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks for optimal accuracy 2
• Never average LMP and ultrasound dates—always prioritize the ultrasound date when first-trimester measurement is available 2

Common Pitfalls to Avoid

• Do not rely solely on LMP when ultrasound is available, as ultrasound provides superior accuracy in the first trimester 2
• Do not use second-trimester ultrasound for initial dating when first-trimester ultrasound could have been performed, as accuracy decreases significantly after 13 weeks 2
• If gestational age changes by ≥2 weeks after ultrasound examination, all previous assessments must be reinterpreted 1
• Screen-positive results should not be reclassified based on dating changes unless the revised gestational age differs by at least 10 days 2