On most major airlines in 2026, a diaper bag does not count toward your carry-on or personal item limit when you are traveling with an infant or toddler. Carriers like American Airlines, Delta, and United generally treat diaper bags as an "essential medical or child-care item," similar to a stroller or a car seat, allowing it to be brought on board for free in addition to your standard baggage allowance. However, this policy is not universal; some ultra-low-cost carriers (ULCCs) may have stricter rules and might require the diaper bag to fit within your purchased allowance or count as your one personal item. It is also important to note that this "free" bag is typically limited to one per child. Even on airlines where it is free, the bag must still comply with standard size restrictions to ensure it fits in the overhead bin or under the seat. Always check the "Traveling with Children" section of your specific airline's website, as these "hidden" perks can save you significant money and stress during your journey.