I smoked for the first 10 weeks of pregnancy - I found out at 5 weeks and gradually cut down until I was smoke free and didn't smoke again until a while after my DS was born.
You can't change the past but I would really, really advise you tell someone that you have been smoking throughout your pregnancy, just in case there are any complications during birth or respiratory problems with your baby once he or she is born (which, to my v limited knowledge, is possible with a baby who has been exposed to smoke in utero). This will allow your baby to have the best care possible if anything should arise, and although I know you fear judgement, you will have to put your child first in this instance.
The main risk with SIDS is what happens after the baby is born, but sadly there are some who will succumb to it even if you do everything right. Smoking around your baby will definitely increase that risk, so it's brilliant that you've stopped now - long may that continue. But if for whatever reason you find yourself unable to stick to it, you must be aware of when and where you smoke, and it'll be too risky to co-sleep if you do smoke (co sleeping is a hotly debated topic in general, I find).
Ways of minimising SIDS is good sleep hygiene for baby, so;
- no blankets, muslins or teddies in Moses basket
- being in the same room as baby when sleeping to listen to changes
- not overheating baby
- use of a dummy
- not falling asleep with baby on sofa or in arms
- placing baby flat down on their back to sleep (ie not in car seat or bouncer)
Alternatively you can use a sensor monitor mat which will alert you if baby stops breathing (sensed through motion). I bought one of these irrespective of the fact I wasn't smoking as I too had huge anxiety re: SIDS and honestly it was a life saver for my mental health for the first 18 months. They are expensive but we still use the monitor to this day (DS is 3) so you get good use and you could always sell it on.
Try not to beat yourself up too much for what has happened. Not everyone can have relaxing, stress free pregnancies. Some people drink, some smoke. It's not advisable obviously and it's not great that it's taken this long for you to stop but the main thing is you are aware and want to change. Just make sure you tell your midwife and if you have a consultant, them too, so that any appropriate help can be given to you and baby it needs must