It's a tricky one... I'm assuming this is your first baby, so you don't yet know how your body settles post partum?
I think I'd be inclined to go shopping now and try dresses on. Realistically, wedding dress shopping with a newborn will be tricky depending on post birth recovery, breastfeeding and/or whether you're willing to leave them with someone else... therefore shopping in December or January would be tricky and even December could be cutting it extremely fine (6 month lead times are not unheard of, although not all suppliers are like that).
Shopping with a small bump might actually be a little helpful in terms of setting expectations for your post partum body... If you spring back into shape quickly then it's easy to have the dress taken in and if you put on a little weight/don't spring back you have an allowance for adjustment. Realistically there will probably be a middle ground (e.g. I was lucky and sprang back into shape, however my rib cage remained slightly larger than before despite being exactly the same dress size/measurements everywhere else...).
When shopping I'd bear in mind that your waistline will have thickened more than you realise (I tried on and bought a relaxed fit dress with buttons ideal for breastfeeding halfway through my last pregnancy, stashed it in the wardrobe until I'd given birth and was then pleasantly surprised at the difference in fit once I'd had the baby... it was so much more flattering... funny that 🤣).
The other thing to bear in mind is whether you're planning to breastfeed. There are ways of getting round it (e.g. going to a different room and half undressing!) but you might want to give yourself the option of a dress that allows easy access, whether that's through simply being able to push the neckline down to "flop a boob out" (look out for fabrics that don't wrinkle or crumple), drop a shoulder or add a concealed zip, popper on a strap, etc. (it's amazing what seamstresses can do). Have a look at the FB page "can I breastfeed in it" and you'll find some inspiration.