ACOG updates its recommendations for estimating gestational age and due date: May 2017

http://www.ultrasoundpaedia.com/normal-1sttrimester/

Accurate gestational dating of pregnancy is very important for optimal maternal and fetal outcome.Throughout pregnancy decisions like ordering and interpreting lab tests, determining fetal growth and performing intervention to prevent preterm births or post-term pregnancies and associated morbidities are based on accurate dating.

Estimated Due Date (EDD)and current gestational dating should be documented on medical records and discussed with the patient as early as possible based on dates of Last Menstrual Period(LMP) and earliest available ultrasound in pregnancy.

A first trimester ultrasound can improve the accuracy of predicted EDD, even if date of LMP is known. Many women have irregular cycles, or falsely recall the date of LMP or have irregular ovulation, which is not considered when calculating the EDD by traditional method.

The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal–Fetal Medicine make the following recommendations regarding the method for estimating gestational age and due date:

A first trimester ultrasound of the embryo (up to and including 13 6/7 weeks of gestation) is most accurate for calculating or confirming the gestational age. Measuring the crown–rump length (CRL) is accurate up to ±5–7 days and it should be the mean of three discrete CRL measurements when possible.

If the pregnancy is the result of successful ART procedure, then the EDD is calculated based on age of embryo and day of transfer.

As soon as LMP and results of first trimester ultrasound are known, EDD should be calculated and recorded in medical records. It should also be told to patient and discussed with her.

Second trimester ultrasound for gestational dating (14 0/7 weeks to 27 6/7 weeks of gestation) is more complex and introduces more complexities and variability. It incorporates multiple parameters like the biparietal diameter and head circumference, the femur length and the abdominal circumference. The accuracy decreases as we progress further into second trimester, with a variability of ± 10–14 days at the end of second trimester.

Gestational age assessment by ultrasonography in the third trimester (28 0/7 weeks of gestation and beyond) is the least reliable method, with an accuracy of ± 21–30 days.

Pregnancies without an ultrasound examination that confirms or revises the EDD before 22 0/7 weeks of gestational age should be considered suboptimally dated and managed accordingly.

This document replaces the Committee Opinion Number 611, (October 2014).

The full text of the committee opinion can be accessed here.