Cornstarch, like talcum powder, can cause respiratory problems even in small amounts. Inhaled cornstarch powder can damage a baby's growing lungs. If you do decide to use cornstarch powder be sure to keep it away from the infant's face, as well as your own. Pour it into small amounts and avoid breathing it in.
The National Institutes of Health (NIH) recommends against use of cornstarch at any time as it can worsen a diaper rash with candidal infection.