An adaptor, system, and method for collecting for a breast pump. A breast milk bag has a hole adjacent an end margin. The bag can have a reclosable seal and a zone of weakness for separating a sealed portion from a header. An adaptor has inlet end and an outlet end and defines a flow passage. The inlet end is securable to the outlet of the breast pump so liquid therefrom can flow through the flow passage. A slot extends through the adaptor wall to at least partially define a retainer for extending through the hole to mount the bag on the adapter. In a method of use, the bag is secured to the adaptor using the retainer, filled with breast milk, sealed while secured to the adaptor, and the sealed portion of the bag can be removed by tearing the bag along the zone of weakness.
