The most reliable fetal parameter for estimation of gestational age in first trimester is (LQ)
|B||G sac diameter|
a. Gestational age (GA) in the first trimester is usually calculated from the fetal crown-rump length (CRL).
b. This is the longest demonstrable length of the embryo or fetus, excluding the limbs and the yolk sac. The correlation between CRL and GA is excellent until approximately 12 weeks.
c. No sex or race differences are appreciable, but maternal characteristics, such as age and smoking, may have a significant effect beyond 10 week gestation.
d. The GA estimate has a 95% confidence interval of plus or minus 6 days, and it is most accurate between 7 and 10 weeks of pregnancy.
e. Transabdominal ultrasonography may underestimate gestational age by an average of 1.6 days compared with transvaginal ultrasonography.