The summer liner is best used once your little one has outgrown the newborn insert. While they’re still using the newborn insert, it should be used on its own, so your baby gets the perfect fit and the highest level of safety. The insert’s soft, breathable fabric helps keep them cool in summer and snug in winter, so they stay comfortable whatever the weather. If you’d like, you can place the summer liner underneath the insert — but the Hydro Protect liner is already designed to create a soft, cocooning space that keeps newborns happy on every journey.