It’s the age-old question: do “one-size” diapers truly fit on newborns? We had a significant newborn diaper stash to test drive for blog reviews, so I did not try a one-size option on Little Miss Cub until she was almost two weeks old. That said, at that age, she weighed the same as many newborns (probably about 7.5 lbs). For sure, there are no one-size diapers that I know of that are suitable for use when the umbilical cord is still attached (correct me if I’m wrong!). Here’s my post about bridging that gap using bridge that gap using flour sack towels and newborn covers.
Earliest Fitting One-Size Diapers (7-8 lbs)
At 7.5 lbs, and I can envision how they would’ve worked even sooner, both Funky Fluff and TotsBots one-size diapers fit and performed very well. We’ve been using them since.
I used TotsBots V4s on Cub overnight when he was 2 years old, so you can imagine that they are very absorbent for a newborn! Little Miss Cub can easily wear a V4, even without the included booster, on long excursions without leaks. (Up to as much as six hours, but it would be unusual to leave her in her diaper so long during the day.) Overnight, the V4 can be boosted with its included booster, or a trimmer one if necessary, and I don’t change her until morning (8-10 hours). To get a nice, snug fit around her legs, I am sure to pull the sides of the waist up before I secure the velcro. Our TotsBots V3 (minky interior) also fit perfectly, but are not as absorbent.
Both the Funky Fluff 2.0 (in Stay Dry and Bamboo) and the newly released Lux also fit very nicely under 8 lbs. They fit snugly around her legs, even when they were not yet chubby from mama milk! I tend to use the stay dry shells as AI2s (bamboo insert or DeLux hemp/bamboo insert on top) because I find that to be the most leak proof option, and I may or may not bother stuffing the insert inside the bamboo shells.
Fully Functional by 9-10 lbs
By the 10 lb mark, all of the BumGenius models fit nicely (I found them too loose at the legs when she was slightly slimmer). I haven’t had leaks using the 4.0, and I always stuff it with our Öko hemp trifolds. I’ve had leaks with the Freetime and Elemental, however, and I do find they don’t fit as nicely as the 4.0 at this point.
Our Rumparooz is an awesome fit (better than BumGenius at this point) right now, and I use the full-size insert inside. It’s fluffy, for sure, but the fit is great and the absorbency is excellent. The photo below she would be closer to 9 lbs than 10.
GlowBugs sent Little Miss Cub a gift of some of their diapers, and I hadn’t been planning on trying them out til much later, since they looked huge. Well colour me WRONG! They would most likely have fit as early as our Funky Fluff. 
GlowBugs have not one but TWO sets of crossover snaps. I will be telling you all about GlowBugs in a review still to come!
A totally new brand to us is Little Bee Co. We didn’t receive them early enough to try on her at 8 lbs, but, based on the fit of their Bee Changed Bamboo now, I am confident they would’ve fit when she was even smaller.
The GroVia ONE is also a winner by the 10 lb mark, but definitely using the velcro rather than the snaps. It’s not yet my favourite fit.
Moraki also works great at this size: 
In terms of the one-size diapers in our stash that are still not fitting well, there’s our EBB, Sweet Pea AIO, Blueberry Simplex (whose newborn cousin is still holding up amazingly well) and our SunBaby (we have the “larger” cut).
Which one-size diapers still fit at 35 lbs?
So, of all these one-size diapers, there are only two that I can’t really use on Cub who is now 3 years old and 35 lbs. Rumparooz and TotsBots are too small, but only in terms of rise. He gets a major plumber’s crack! Cub is above-average for height, so that’s something to consider. Give either of these diapers half-an-inch more in the rise and we’d still be able to use them.
Which brand would I recommend for the truest “Birth to Potty” fit?
Based on the awesome fit on both newborn sister and preschooler brother, I’d have to give my recommendation to Funky Fluff. They really do snap down small enough to fit around newborn chicken legs, but fully unsnapped, the rise is tall enough for Cub at about 40 inches tall. I have a review coming up for Glow Bug, and they also fit both Little Miss and Cub!
Which one-size diapers fit best on your newborn?
Check out all of my cloth diaper reviews here.
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