Dear user, welcome to healthcare magic.
The first possibility which must be excluded for a small for gestational age
baby is wrong date calculation. Your dates must be sure and reliable and must be confirmed by a first trimesteric ultrasound
If the calculation is right, the Doppler is normal and the baby is symmetrical, then we have two possibilities: the first possibility which is by far the most common is constitutional, meaning that it is a small healthy baby. The second possibility is presence of an inherent defect in that baby (congenital infection or chromosomal defect) and this is the rare possibility.
It is very difficult to differentiate between the two possibilities and usually the diagnosis is done retrograde after delivery. I see no importance of TORCH screening now.
I suggest you don't worry and just follow up with your doctor.
I wish you good luck and a healthy baby.
Dr. Ahmed Bahaa.