At 28 weeks pregnant my wife was found to have low ferritin (9 g/l) but normal hgb (123 g/l). her iron wasn't tested before but hgb was 135 g/l at 11 weeks. can we infer if iron was low before and if there was a risk to baby's brain development?

Not necessarily. We find that the placenta and babies other systems selectively take from mothers body during pregnancy to get what they need while leaving mom depleted. There can be a point, marked by poor baby growth parameters for gestational age where it could be. However, moms body is doing much of the metabolic work for the baby so it rests & grows. This is not that worrisome.

No. But she should take oral iron and multivitamins to replenish her iron stores. The fetus is not at risk from this. Wish your wife good health and a healthy baby!