Determining Gestational Age in the Context of Abortion

Primary Method: First-Trimester Ultrasound

The American College of Obstetricians and Gynecologists recommends first-trimester ultrasound using crown-rump length (CRL) measurement between 8-13 weeks for determining gestational age, providing accuracy within ±5-7 days 2
When CRL measurement is available and shows a discrepancy of ≥5 days from last menstrual period (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 American College of Medical Genetics recommends last menstrual period (LMP) as the most common method for initial gestational age assessment, though it is less accurate than ultrasound 3
LMP assumes regular menstrual cycles and accurate recall of the first day of the last menstrual period 1
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 3, 2

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

Accurate gestational age determination is critical because maternal morbidity and mortality from induced abortion increase significantly with advancing gestation 4, 5
At 11-12 weeks' gestation, the mortality rate is 1.1 deaths per 100,000 abortions, and at 16-20 weeks' gestation, the mortality rate is 6.9 deaths per 100,000 abortions 4, 5
Major complication rates increase with gestational age, from 0.8% at 11-12 weeks to 2.2% at 17-20 weeks 4, 5

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
Gestational age should be expressed as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks for optimal accuracy 2
The ultrasound date should be prioritized when first-trimester measurement is available, and LMP and ultrasound dates should not be averaged 2