They are asking about anything that can impact on development, so anything that guides your child's abilities and difficulties at the time of assessment.
These include - maternal nutrition (only a problem if you were starving really), alcohol exposure during pregnancy, drug exposure during pregnancy (prescribed and non prescribed) , infections during pregnancy, anything which restricts baby's oxygen during pregnancy, any complications at birth. They are trying to rule in or out these things, it may be that none of these things had any impact at all on your son's development or diagnosis.
What you can do for a future pregnancy - generally healthy, healthy weight range, eat well, mild to moderate exercise, take folate/folic acid (higher dose if high BMI), don't drink alcohol in 2ww or when pregnant, keep exercising when pregnant (mild to mod), don't use non prescribed medication, don't drink crazy high amounts of caffeinated or taurine drinks, discuss if prescribed medication has any impact on pregnancy or fetal development with GP or midwifery.
But important to stress that lots of these questions are to rule out things like fetal oxygen deprivation, FASD. Also lots of these things are out of your control. It's likely that none of these things, nothing you did or could do, specifically led to your son having ASD.