Tech said baby's legs were short. How do I know if he is a dwarf?

Femur length. Ultrasound imaging of the fetus can determine femur length with great accuracy. The normal relationship between the bones of the lower leg and femur is 1:1. If the femurs are short, either a constitutionally normally small fetus (frequently offspring of short parents), intrauterine growth restriction (iugr) or a skeletal dysplasia may exist. Normal short fetuses exhibit appropriate interval growth.
If you mean length . of the femur on a fetal ultrasound, ask your OB about its significance & if there are other findings that suggest dwarfism. You may wish to consult a Fetal-Maternal Medicine specialist. Dwarfism can be diagnosed on FUS from 16-32 weeks, depending on the type. Anxiety during pregnancy has its own risks. Don't add to your concerns about advanced maternal & paternal age needlessly; seek an answer.