Which teratogen is commonly associated with low birth weight, subnormal length and head circumference, excitability, and subnormal reflexes?

I believe the answer is Alcohol

Alcohol is one of the most common teratogens. Many mothers don't realize that they are pregnant still consume alcohol until a few weeks of pregnancy. Alcohol has effects on the nervous system and the fetus will keep being susceptible in all three trimesters of pregnancy. 
Fetal alcohol syndrome includes the low birth weight, low head circumference, and many kinds of symptoms related to nervous system.