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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.
