The femur length and the , head circumference and the ac gives you the accurate fetal age? (2nd trimester) .thank you!

Yes.: These measurements (sometimes including the biparietal diameter) generally gives an accurate gestational age (with in 5- 7 days) up to about 20 weeks gestational age. After 20 weeks the margin of error starts increasing.

Fetal measurements: on ultrasound are accurate to about a week. That being said, it is still only an estimate. Only 5% of babies are born on their due date. Hope this helps!