Methods for Estimating the Due Date: Committee Opinion Overview
The accurate dating of pregnancy is essential for optimizing outcomes and is a critical research and public health priority. This article presents the guidelines developed by the American College of Obstetricians and Gynecologists (ACOG), in collaboration with the American Institute of Ultrasound in Medicine and the Society for Maternal–Fetal Medicine, to provide a standardized approach for estimating the gestational age and due date (EDD).
Key Recommendations:
First-Trimester Ultrasound: The most accurate method for confirming gestational age is by measuring the embryo or fetus during the first trimester (up to 13 6/7 weeks).
ART Pregnancies: For pregnancies resulting from Assisted Reproductive Technology (ART), the EDD should be based on the age of the embryo and the date of transfer.
Gestational Age Calculation: The gestational age and EDD should be determined as soon as data from the last menstrual period (LMP) or the first ultrasound are available and documented in the medical record.
Rare EDD Adjustments: Changes to the EDD should be reserved for rare circumstances and require thorough documentation.
Importance of Accurate Pregnancy Dating
Accurately establishing an EDD early in prenatal care is critical for scheduling appropriate interventions, managing fetal growth, and preventing preterm and postterm births. Precise dating also supports research efforts and ensures consistency in vital statistics.
Traditional LMP Method
The conventional method of dating pregnancy involves calculating 280 days from the first day of the last menstrual period (LMP). However, this approach assumes a regular 28-day cycle and does not account for inaccuracies in LMP recall or variations in ovulation.
Clinical Considerations by Trimester
First Trimester (Up to 13 6/7 Weeks)
Ultrasound measurements during this period have an accuracy of ±5–7 days.
The crown–rump length (CRL) is the preferred metric for establishing gestational age, especially before 9 weeks, with adjustments made when discrepancies of more than 5 days arise between LMP and ultrasound dating.
Second Trimester (14 0/7 to 27 6/7 Weeks)
Gestational age is assessed using a composite of biometric measurements, including biparietal diameter, head circumference, femur length, and abdominal circumference.
Accuracy decreases to ±7–10 days, and adjustments to the EDD are made when the ultrasound differs by more than 7–14 days from LMP estimates.
Third Trimester (28 0/7 Weeks Onwards)
Ultrasound accuracy declines to ±21–30 days, making it the least reliable for dating.
Redating in the third trimester is generally discouraged unless discrepancies exceed 21 days.
Conclusion
Accurate dating enhances pregnancy management and care. The EDD should be determined early using ultrasound and LMP data, and subsequent adjustments should be infrequent, based on clear clinical evidence. Establishing a standardized approach across institutions supports consistent clinical outcomes and research efforts.
