If the duvet you have is a summer duvet with a rating of 4.5 TOG it may be worth upgrading to a 10.5 TOG all weather or a 13.5 TOG for winter only.
The Ultimate Guide to Choosing the Right Sleepwear for Your Baby
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Do clothes underneath the swaddle have to have a tog rating aswell? : r/BabyBumpsandBeyondAu
What is a TOG rating?
What is a TOG rating?, Temperature Chart
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These thermal socks have been designed to keep your feet warm during the cold winter month. They have been tested to BS4745 Standard, achieving a Tog
2.13 TOG Rating Thermal Socks For Men and Women Keep Feet Warmer Socks - Black, 10 - 13, 1 Pair
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TOG Rating Guide & Temperature Chart for Babies
TOG Guide – Tealbee