Can you really measure head and femur lenghts in ultrasound? What does that mean? I had an ultrasound done on my 26 week old baby and the doctor said the femur length measued 26 weeks, but the head an abdomen were 29 weeks. Is this a sign of a deformity

Depends. Measurements are usually accurate. Discuss your concerns with your obstetrician. There are variations of normal.
This. This finding probably does not represent a deformity or birth defect. Everyone is built a little differently and so are babies. The ultrasound technologist measures the baby's head circumference, head diameter, abdomen, and femur length (occasionally other measurements are also included). These measurements are compared to known averages in order to estimate a baby's age and make sure he/she is growing appropriately. However, averages also come with an acceptable range of variability. The dates that you have listed certainly could be within the normal range of variability, especially if no other abnormalities were identified during the ultrasound exam and your pregnancy has been progressing normally.