Muslin is a loose, open weave, and that weave is what makes it breathable and quick drying. It also means the fabric relaxes a bit with wear and gets softer with every wash. A piece that fits well on day one will feel roomier and more relaxed after a month of washing, so it does not usually shrink or tighten the way some fabrics do.
Because of that, sizing up is really about how long you want a piece to last, not about compensating for shrinkage. If you want a playsuit or set to fit through a growth spurt or into a new season, going up one size gives you a few extra months of wear. If you want the current fit and don't mind swapping sizes more often, true to size works fine. Our sizes run roughly newborn through 4 years, so check the size guide against your child's current measurements before you decide.
Wash cold on a gentle cycle and tumble dry low or hang to dry, and skip the fabric softener and high heat, since heat is what can affect the fit over time, not the fabric's natural relaxing. Creasing is normal for muslin and comes out with a wash or a light iron. If you do size up or down and it is not right, we take unworn returns within 30 days so you can swap it out.