They look allergic in nature . If they are remaining limited to the areas in the picture they may be related to the diapers so use them sparingly. Keep the areas dry and frequently exposed to the air for complete drying .  As you say that they subside on their own and are not troublesome for the baby there is no need of giving Atarax too . Does someone else in the family have a problem of allergy like skin allergy , bronchial allergy etc ? Then this may be something called atopic dermatitis in which case finding what causes these rashes would be the key to prevention