Best Time to Date a Pregnancy
The optimal time to date a pregnancy is during the first trimester between 8-13 weeks using crown-rump length (CRL) measurement, which provides accuracy within ±5-7 days and represents the gold standard for pregnancy dating. 1
First Trimester Dating: The Gold Standard
Perform dating ultrasound between 8-13 weeks of gestation using CRL measurement whenever possible. 1, 2 This timing provides the highest accuracy, with precision as good as ±1.5 days compared to true gestational age. 1, 2 The CRL measurement should capture the greatest length from crown to rump, excluding limbs and yolk sac. 1
Why First Trimester is Superior
- CRL measurement at 8-13 weeks achieves ±5-7 days accuracy, significantly better than any other dating method. 1, 2
- Accuracy progressively declines after 13 weeks, making early dating critical for optimal pregnancy management. 2
- First trimester ultrasound is more accurate than last menstrual period (LMP) dating and should always supersede LMP when available. 1, 2
Second Trimester Dating: Acceptable but Less Accurate
If first trimester dating was missed, perform composite ultrasound dating at 18-22 weeks using multiple fetal measurements (biparietal diameter, head circumference, abdominal circumference, femur length), which provides accuracy within ±10 days. 3, 1, 2 This represents a significant decline in precision compared to first trimester CRL.
For pregnancies at ≥14 weeks when LMP is uncertain, biparietal diameter (BPD) measurement alone is specifically recommended as it rules out anencephaly and improves detection of open spina bifida. 3, 1, 2
Critical Decision Rules for Dating Discrepancies
When a discrepancy of ≥5 days exists between LMP and first trimester CRL dating, always adopt the CRL-based gestational age for all clinical decision-making. 1, 2 Never average the two dates—this is a common error that compromises accuracy. 1, 2
Document both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date as the definitive gestational age. 1, 2
Special Populations
For assisted reproductive technology (ART) pregnancies, calculate gestational age by adding 14 days to the number of completed weeks since fertilization, as the exact fertilization date is known. 1 This provides highly accurate dating without requiring ultrasound confirmation.
For multiple gestations, perform first trimester dating when CRL measurement is between 45-84 mm, ideally at the time of nuchal translucency evaluation. 3 This timing allows simultaneous assessment of chorionicity, dating, and nuchal translucency screening.
Clinical Implications of Accurate Dating
Accurate dating is essential for appropriate timing of aneuploidy screening and neural tube defect screening, as serum screening results are highly dependent on precise gestational age. 1, 2 Maternal serum AFP levels increase by 10-15% per week in the second trimester, making even small dating errors clinically significant. 1
Express gestational age as weeks and days (e.g., 15 weeks 5 days) rather than rounded weeks for optimal accuracy in clinical decision-making. 1, 2
Suboptimally Dated Pregnancies
Pregnancies without ultrasound examination confirming or revising the estimated due date before 22 0/7 weeks should be considered suboptimally dated. 4, 5 For these pregnancies, consider interval ultrasound assessment 3-4 weeks after initial study to improve gestational age estimation. 4
Common Pitfalls to Avoid
- Never average LMP and ultrasound dates—always use the ultrasound date when first trimester CRL is available. 1, 2
- Do not delay dating ultrasound beyond 13 weeks thinking second trimester dating is equivalent—accuracy declines significantly. 2
- Avoid using obsolete terminology such as "fetal pole" or "embryonic pole" in documentation. 2
- Do not change the established due date after 22 weeks except in rare, well-documented circumstances. 4, 5