Determining the Due Date in Pregnancy

Optimal Dating Hierarchy

The American College of Obstetricians and Gynecologists recommends determining the due date using first-trimester ultrasound crown-rump length (CRL) measurement when available, as this is the most accurate method with precision within ±5-7 days 1
CRL measurement between 8-13 weeks provides the highest accuracy (within ±5-7 days), making it the gold standard for pregnancy dating 1
When a discrepancy of ≥5 days exists between LMP and CRL dating in the first trimester, adopt the CRL-based gestational age for all clinical decision-making 1
The accuracy at first ultrasound can be as precise as ±1.5 days compared to true gestational age 1

Composite ultrasound dating using multiple fetal measurements between 18-22 weeks is accurate to within 10 days, as recommended by the American Institute of Ultrasound in Medicine 2, 1
Biparietal diameter (BPD) measurement alone is recommended at ≥14 weeks when LMP is uncertain or discrepant, as it rules out anencephaly and improves detection of open spina bifida, according to the American College of Medical Genetics 2, 1

Calculate gestational age by adding 14 days to the number of completed weeks since fertilization, providing highly accurate dating since the exact fertilization date is known, as stated by the American Society for Reproductive Medicine 3, 4

Record both LMP-based and ultrasound-based dates in the medical record, but use the ultrasound-based date for clinical decisions, as recommended by the American College of Obstetricians and Gynecologists 1
Express gestational age as weeks and days (e.g., 15 weeks 5 days) or decimal weeks (15.7 weeks) rather than rounded weeks for optimal accuracy, according to the American Institute of Ultrasound in Medicine 2, 1

Accurate dating is crucial for appropriate timing of Down syndrome and neural tube defect screening, as stated by the American College of Obstetricians and Gynecologists 1
Serum screening results are highly dependent on precise gestational age, and even small dating errors can affect interpretation, according to the American College of Medical Genetics 1
Maternal serum AFP levels increase by 10-15% per week in the second trimester, making accurate dating essential, as recommended by the American College of Medical Genetics 2

Never average LMP and ultrasound dates - always use the ultrasound date when available in the first trimester, as recommended by the American College of Obstetricians and Gynecologists 1
Do not use package insert medians for AFP screening without establishing laboratory-specific normative data, according to the American College of Medical Genetics 2
Ensure proper CRL measurement technique by measuring the greatest length from crown to rump, excluding limbs and yolk sac, as stated by the American College of Obstetricians and Gynecologists 1

The American College of Radiology recommends performing composite ultrasound dating at 18-22 weeks using multiple fetal measurements, which provides accuracy within ±10 days, for pregnancies where first trimester dating was missed 5
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, according to guidelines from the American College of Radiology 5

For multiple gestations, the American College of Radiology recommends performing first trimester dating when CRL measurement is between 45-84 mm, ideally at the time of nuchal translucency evaluation, to allow simultaneous assessment of chorionicity, dating, and nuchal translucency screening 6, 7

The American College of Medical Genetics recommends that screen-positive results should not be reclassified to screen-negative based on dating changes unless the revised gestational age differs by at least 10 days, to prevent inappropriate reclassification while allowing correction of significant dating errors 8
Proper CRL measurement technique is crucial, measuring the greatest length from crown to rump, excluding limbs and yolk sac, to ensure the accuracy advantage of ultrasound dating 9
The American College of Medical Genetics suggests that biparietal diameter (BPD) alone should not be used for initial dating in the first trimester, as CRL remains superior for establishing gestational age in early pregnancy 9, 10

The American College of Obstetricians and Gynecologists recommends using ultrasound dating as the official gestational age when CRL is correctly measured between 11-13 weeks, regardless of discrepancy with LMP, with an accuracy of ±5-7 days 11

Accurate dating is essential for prenatal screening because biochemical markers change rapidly with gestational age, with maternal serum AFP increasing 10-15% per week in the second trimester, and PAPP-A levels rising rapidly in late first trimester 11