What estimated due date (EDD) should be used when the second trimester ultrasound provides an EDD 2 weeks earlier than the original EDD in a patient with an unknown last menstrual period (LMP)?

From the Guidelines

When a second trimester ultrasound provides an estimated due date (EDD) that is 2 weeks earlier than the original EDD in a patient with an unknown last menstrual period (LMP), the ultrasound-based EDD should be used. In this scenario, since there is no reliable LMP for comparison, the second trimester ultrasound dating becomes the most accurate method available for establishing the EDD. Second trimester ultrasounds (typically performed between 14-20 weeks) can estimate gestational age with reasonable accuracy, usually within 7-10 days, as supported by 1. The earlier in pregnancy an ultrasound is performed, the more accurate it is for dating purposes. Without a known LMP, there is no other reliable clinical information to contradict the ultrasound findings. Using the ultrasound-based EDD is important for appropriate pregnancy management, including timing of screening tests, monitoring fetal growth, and planning for delivery. This approach helps prevent complications related to incorrect dating such as unnecessary interventions for presumed post-term pregnancies or inappropriate timing of indicated preterm deliveries. Key points to consider include:

• Ultrasound measurement of crown-rump length (CRL) in early pregnancy provides an accurate estimate of gestational age, but in the second trimester, composite ultrasound dating is accurate to within 10 days 1.
• The use of biparietal diameter (BPD) measurement alone is recommended for pregnancies at 14 weeks gestation or later, or when the LMP is uncertain or discrepant with physical examination 1.
• Although other studies discuss the importance of ultrasound in pregnancy, such as 1 and 1, the most relevant information for this specific question comes from 1, which directly addresses the accuracy of ultrasound dating in the second trimester. However, it's worth noting that the most recent study 1 does not directly address the question of EDD estimation but highlights the critical role of ultrasound in pregnancy management. Given the information provided and prioritizing the most recent and highest quality study, the recommendation to use the ultrasound-based EDD stands, as it is based on the best available evidence for gestational age estimation in the absence of a reliable LMP.

From the Research

Determining the Estimated Due Date (EDD)

When the second trimester ultrasound provides an EDD 2 weeks earlier than the original EDD in a patient with an unknown last menstrual period (LMP), the following points should be considered:

• The American College of Obstetricians and Gynecologists recommends that the estimated due date should be determined as soon as data from the last menstrual period, the first accurate ultrasound examination, or both are obtained 2.
• Ultrasound dating is generally more accurate than LMP dating, especially when the LMP is uncertain 3, 4.
• A study found that ultrasound was superior to certain LMP in predicting the day of delivery by at least 1.7 days 3.
• When a discrepancy between menstrual and ultrasound dating occurs, a simple formula called the rule of eights can be used to determine a final estimated date of delivery 5.

Choosing the EDD

Based on the evidence, it is recommended to use the EDD provided by the second trimester ultrasound, as it is likely to be more accurate than the original EDD based on LMP 3, 4.

• The ultrasound EDD should be used as the best obstetric estimate for the purpose of clinical care and should be recorded on the birth certificate 2.
• Subsequent changes to the estimated due date should be reserved for rare circumstances, discussed with the patient, and documented clearly in the medical record 2.