Britax used to allow babies to be up to an inch under the first slots, but has revised their rules to state that an infant must be at or over the first slots in order to use the seat.
honestly, I don't know why they say the seat can be used from 5 pounds. I can not imagine ANY baby who weighs only 5 pounds being anywhere NEAR big enough for this seat.
As the pp said, almost no babies fit into a marathon at birth. Certainly no small babies. Even very large 9-10 pound babies normally don't fit until 2-3 weeks when they have that first growth spurt. Normal 7-8 pound babies generally need 1-3 MONTHS before they fit, and smaller or shorter babies can take up to 6 months or more to fit into a marathon.
My very large 9 pound, 21 inch child would not have fit a marathon until she was a few weeks old.
My very tiny 5.5 pound, 17 inch long son did not truly fit properly until he was about a year old, although at about 8 months he began using the 10 inch bottom slots, even though they were still a "hair" above his shoulders, and I should not have technically been using the seat yet.
as an aside, no 65 pound kid is going to fit into a marathon either. Most kids become too tall (outgrow by height) the marathon when they are 35-50 pounds.
Actually, for these reasons, the Britax seats have fallen out of favor amongst CPS professionals. They do not fit newborns, so you HAVE to use anitehr seat first...then, they have only a 35 pound rearfacing limit (many seats now go to 40-45 seats rearfacing) and then forward facing, they only have about 16.5 inch top slots (many seats have 17-18.5 top slots now) so they are outgrown by height years earlier than many other seats. Also, they are extremely expensive, especially considering the reduced use you get out of them.
Again...i can't imagine a small newborn fitting into a marathon. The simple MATH precludes the possibility. The bottom slots are at 10 inches. A small newborn is going to be 18, 19, maybe 20 inches long, right? A significant portion of that is going to be that big ole newborn head, and some of it is going to be legs. It's simply not possible that for a human newborn who is 19 inches total, that ten of those inches are going to be torso. It's impossible. Is what I'm saying making sense?
now..I did think of one possibility, and that is if maybe a parent didn't know any better, and used a big thick snowsuit, or "bundleme" or other aftermarket baby product which might have artificially increased the size of the baby, the baby might have "seemed" to fit in the marathon, but sadly, was dangerously, improperly, unsecured. Obviously, we are trying to get the word out and educate parents against using these dangerous products, but it still happens, a lot.