Under most major airline policies in 2026, a diaper bag does not count toward your standard carry-on limit. Most full-service carriers (like Delta, United, and British Airways) allow passengers traveling with an infant to bring one diaper bag in addition to their standard "Personal Item" and "Carry-on Bag" for free. This is considered an "essential medical/infant item" similar to a stroller or a breast pump. However, this policy is less consistent with ultra-low-cost carriers (like Spirit or Ryanair), who may require the diaper bag to fit inside your paid luggage or be counted as your one allowed item. To ensure a smooth boarding process, always check the "Traveling with Children" section of your specific airline's website. If you are flying on a lap-infant ticket, you are almost always granted this extra bag as a courtesy to accommodate the necessary supplies for the baby during the flight.