Hi ying!
DD was on the smaller side when born (5'14 or 2660g) so no MCN fitted her. But as soon as my small nappies (bubblebubs, kidz in coth, berry plush) started fitting, so did my blueberry and bumG pockets.
I found the blueberry great, obviously a bit more bulky than a sized nappy, but still a good fit. The leg elastic is great, no poospolsions out of my blueberry!
DD is still on the smallest setting, although the tabs don't cross over anymore. But as we discovered last night, she is not the tallest baby in the world

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I used bumg and blueberry's on my DD from about 8weeks i think. she was only 5pound14 so no MCN that i had fitted, i used flats until then. But the blueberry fitted well, great poo-tainment. Not as trim as my S bubblebubs or berryplush, but fine all the same. She is still on the s setting