Pregnancy Dating and Gestational Age Calculation

Introduction to Gestational Age Calculation

The American College of Obstetricians and Gynecologists recommends using early ultrasound (preferably first trimester) as the primary method for gestational age calculation, with an accuracy of estimating gestational age within 5-14 days 1
In assisted reproductive techniques, gestational age is calculated by adding 14 days to completed weeks since fertilization, providing accurate gestational age assignment when available 2

Ultrasound performed before 24 weeks is considered the most accurate method for gestational age calculation, with an accuracy of estimating gestational age within 5-14 days 1
First trimester ultrasound is generally considered the most accurate method for gestational age calculation, as suggested by the American College of Obstetricians and Gynecologists 1

The American College of Obstetricians and Gynecologists recommends using early ultrasound (preferably first trimester) as the primary method, considering last menstrual period (LMP) if early ultrasound is not available, and relying on ultrasound dating if significant discrepancy exists between LMP and ultrasound dates (>7 days) 1
For pregnancies resulting from assisted reproductive technology, the American College of Obstetricians and Gynecologists recommends calculating gestational age from fertilization date plus 14 days 2

Accurate dating is crucial for appropriate management of pregnancy, particularly for timing interventions like antenatal corticosteroids, planning delivery in high-risk pregnancies, and correctly classifying births as preterm or post-term 1
The normal length of a pregnancy is 40.0 weeks (280 days) from the first day of the last menstrual period, with the term period ranging from 37 weeks 0 days to 41 weeks 6 days 3
Gestational age is calculated from the first day of the last menstrual period (LMP) and is reported as completed weeks and days, also known as menstrual age or clinical age 3

The Society of Radiologists in Ultrasound defines the following pregnancy subcategories:

Pregnancy Subcategory	Gestational Age Range
First trimester	≤13 weeks 6 days
Term pregnancy	37 weeks 0 days to 41 weeks 6 days
Early term	37 weeks 0 days through 38 weeks 6 days
Full term	39 weeks 0 days through 40 weeks 6 days
Late term	41 weeks 0 days through 41 weeks 6 days
Post-term	≥42 weeks 0 days [3]