For the trip home, a newborn wears what the app gives a newborn outside, with the car seat's rule on top: no coat or snowsuit under the harness. In the cold, a blanket goes over the buckled straps where the jacket would be.
What the app suggests for a newborn in a car seat, from the skin out:
27–30°C: short-sleeve bodysuit and sun hat. Hot — as little as possible, plus a sun hat and shade. 26°C: short-sleeve bodysuit, pants and sun hat. Warm — light single layers. 25°C: short-sleeve bodysuit, pants, socks and sun hat. Warm — light single layers. 22–24°C: short-sleeve bodysuit, pants and socks. Warm — light single layers. 21°C: long-sleeve bodysuit, pants and socks. Mild — add a light mid layer. 17–20°C: long-sleeve bodysuit, pants, socks and light sweater. Mild — add a light mid layer. 16°C: long-sleeve bodysuit, pants, socks, light sweater and warm hat. Mild — add a light mid layer. 15°C: long-sleeve bodysuit, pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 14°C: long-sleeve bodysuit, warm pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 12–13°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 8–11°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps and warm hat. Cold — warm layers under the straps, a blanket over them, and a hat. 3–7°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and mittens. Cold — warm layers under the straps, a blanket over them, and a hat. −15 – 2°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and warm mittens. Freezing — warm layers under the straps, a blanket over them, hands covered.
The American Academy of Pediatrics advises against bulky coats and snowsuits under car seat straps. Buckle the harness snug over thin layers, then lay the blanket over the top, never between your baby and the straps.
The app dresses a newborn as if it were a couple of degrees colder than for an older baby, so the hat comes on sooner. That is Baby Layers' own step.
Once the car is warm, take the hat and the blanket off. A newborn in a warm car under a winter blanket gets hot quickly.
Before the day, Nurtura Daily's car seat installation checklist goes through fitting the seat and the harness, step by step.
Indoors, or in a warm car, bus or train, take the hat and outer layers off.
Check the neck — warm but not sweaty means just right.