CRL vs. LMP Dating
Discrepancy Analyzer
Does your first-trimester ultrasound due date differ from your last menstrual period? Apply the official ACOG Committee Opinion No. 700 decision tree to determine whether your medical due date should be redated.
Discrepancy Inputs
Compare period dates against first-trimester ultrasound CRL
Official Clinical Source: Last Menstrual Period (LMP)
iClinical Decision Analysis
The discrepancy is 4 days, which is within the acceptable ACOG clinical threshold of ≤ 5 days. Under standard obstetrical protocols, the menstrual LMP due date remains the official clinical baseline.
ACOG Practice Guidelines:
- ✓Continue using your menstrual cycle EDD of Fri, April 16, 2027.
- ✓First-trimester ultrasound CRL has a tight confidence margin of ±3 to 5 days, eliminating errors caused by late or irregular ovulation.
- ✓Once an official first-trimester EDD is confirmed in your medical records, it must remain fixed throughout the remainder of pregnancy.
ACOG Committee Opinion No. 700 Reference Standard:
| Gestational Age Range | CRL Range | Discrepancy Threshold | Official Action |
|---|---|---|---|
| ≤ 8 weeks 6 days | ≤ 22 mm | > 5 Days | Redate to Ultrasound EDD |
| 9w 0d to 13w 6d | 23 to 84 mm | > 7 Days | Redate to Ultrasound EDD |
Why Accurate Pregnancy Dating Matters Clinically
Clinical rationale behind the American College of Obstetricians and Gynecologists (ACOG) redating criteria.
1. Avoiding Unnecessary Premature Inductions
If a woman ovulated on cycle Day 24 instead of Day 14, relying on menstrual dates will falsely portray her baby as 10 days older than biological reality. When calendar dates reach 41 weeks, her obstetrician might recommend medical labor induction when the baby is actually only 39 weeks and 4 days old. Accurate first-trimester CRL dating prevents unnecessary iatrogenic interventions.
2. Preventing Unrecognized Post-Term Deliveries
Conversely, if a woman ovulated earlier than Day 14, menstrual dating will underestimate fetal maturity. Missing a true post-dates pregnancy (beyond 42 completed weeks) increases risks of placental senescence, oligohydramnios (low amniotic fluid), and meconium aspiration syndrome. Accurate CRL dating safeguards maternal-fetal outcomes.
3. The Golden Rule: Never Redate After the First Trimester
Both ACOG and the Society of Obstetricians and Gynaecologists of Canada (SOGC) explicitly mandate that once an official first-trimester due date is determined, it must never be altered based on subsequent second- or third-trimester scans. Late ultrasounds exhibit wide biological variation (±14 to 21 days margin of error) due to parental genetics and fetal growth patterns.