My nine-month baby had a high platelet count (424 thou/ul) in her last 'well-baby' check up. is that really bad?

Nope!: Platelets are a blood component whose number varies dependent on a huge variety of things. They are a component that we in medicine call "acute phase reactants". Their number can go up for any number of reasons, such as even if you are fighting a cold, healing a sore on your leg, having an immune response to shots, making more red blood cells because your baby is growing so fast. Not to worry!

Agree with Dr. M: Iron deficiency can cause elevated platelet counts. Ask your doctor about giving iron supplement to your baby. Wish you and your baby good health!