I assumed that the first implantations occur "lower" in the uterus. One reason I assumed that is that in Smudge's (the Ragdoll) litter, the mom went into labor on April 4, and had a stillborn, almost hairless very small kitten. Then stopped labor until 2 days later, on April 6, and had 5 more. They were not premature looking, but they were a "bit" small. So I was thinking that the later conceived kitten was more toward the "outside."