In general a winter sleeping bag will be 3.5 tog with the rest 2.5 tog but as to what to use depends on the temperature in your house. When my dd was 6 months we struggled to get our house above 13 degrees despite the heating being on so she was also in a cardigan with a blanket on top. Grobag website is good for checking what tog.
Be aware that many sleeping bags have a minimum weight so a newborn has a good chance of being too small (they can slip down if head and arm holes aren't tight enough). I'm also due at Xmas, I'm not planning on using sleeping bags for the first few weeks, I prefer blankets at that stage.
We've got a new build house so double glazed, good insulation. 2.5 tog was fine for our january baby. If your house gets really cold overnight you might need a 3.5. Then there are 1 or even 0.5 tog for the summer.
As pp said they have a lower weight limit for baby - we used the Grosnug from about 10 days old with no trouble as we had a wriggly baby and blankets turned into a real faff.