Among recognized teratogenic/fetal risk factors during pregnancy, which is generally considered the LEAST directly established risk for a specific major fetal malformation, compared with the others listed?

Alcohol, tobacco, and folic acid deficiency each have well-established, specific associations with particular major fetal malformations or significant developmental risks (fetal alcohol syndrome, low birth weight/oral clefts, and neural tube defects respectively). Iron deficiency (maternal anemia), while a genuine risk factor for general pregnancy complications such as prematurity and low birth weight, has a less direct/specific association with a particular structural fetal malformation compared with the other three.