How do you establish gestational age for a fetus with iugr with no known lmp?

Why assume IGR: Gestational age is initially an estimate based on last normal menses and or a ultrasound. Later ultrasound and other factors align to improve the value of estimated gestation. A fetus whose menstrual estimate is further along than the ultrasound measurements might be considered a IUGR. but the term is more accurate when applied to a newborn whose features will verify gestation.