In the absence of a first trimester ultrasound and other information for dating the pregnancy (e.g., when the date of the lastmenstrual period isunknown or cycles are irregular), ultrasound
head measurements must be chosen to estimate gestational age (head circumference or biparietal diameter before 18 weeks) (grade B).When gestational age is still uncertain above 22 weeks, repeating
the biometric measurements 15–20 days later makes it possible to reassess the growth process and sometimes to estimate gestational age more accurately (Professional consensus).