The baselayer is the most important layer on your baby since it sits against the skin. It should fit snugly, wick away moisture, be highly breathable, and feel warm even if it gets wet.
If you can’t remember all that, just remember this: baselayers should never be cotton. Cotton is a buzzkill because it holds sweat against the skin. Sweat cools quickly and in wintry conditions, the constant cold moisture can lead to hypothermia. For that reason alone, don’t skimp in this area.
In the grand scheme of things, this is the least important layer to “get right,” so save some money here by using what you have (even a simple wool sweater works) or scouring consignment shops and thrift stores.
The objective is just to find something that keeps your baby warm. If you have a toddler who’s actively running around, you can keep this layer stashed in a pack and only put it on when he stops moving (otherwise you’ll end up with a damp sweaty kid).
This crucial layer in the baby sandwich protects your little one from the elements. It fends off whatever wild weather that winter throws at you — be it rain, snow, or hail — and keeps it away from the body and the mid layer so your baby stays dry and warm.