One of the things we have been massively pedantic about in our product design and sampling is to have our sizes be as true-to-size as possible. This is also important to ensure the longevity of the wear of each garment per child.
As noble as this sounds it is one of the most difficult things in the world to accomplish for a very simple reason: not any two divinely unique babies are alike!
Our onesies and culottes all feature draw-strings that enable you to buy slightly ahead of size and let the little ones grow into them. But it really depends on what fit you desire straight out of the box.
That being said - please use this sizing chart as a rough indication of accurate sizing - also approximately referencing height and weight. Using height, weight and age as a trifecta we are certain you will be able to purchase with confidence and enjoy your Upstream Baby experience!