Mog there are broadly two types of dental abscess - periodontal , ie pertaining to the gum,(this includes the swelling you can get around an impacted wisdom tooth, and periapical abscesses.(at the tip of the tooth root with the source being a dead or dying nerve inside the tooth). When people talk about having an abscess they usually mean the latter, and that's what you might have.
Periodontal abscesses can self heal , with good oral hygiene, the use of mouthwash such as corsodyl, and in severe cases, antibiotics. Unless the affected tooth is very mobile or impacted the tooth itself can stay put without needing treatment.
Periapical abscesses on the other hand won't go away without treatment (ie a root filling if you want to save the tooth, an extraction if you don't)However not all abscesses are painful so the pain may come and go. Lack of pain does not necessarily mean lack of abscess but it does mean you could delay treatment .
You could indeed get antibiotics for it in this late stage if simple painkillers didn't keep the pain at bay. However there is no real reason not to get a root filling while pregnant (other than the not wanting any additional hassle of any kind when pregnant!)
Root fillings are often done in two stages , with the root canal getting cleaned out at the first stage, and a medicated dressing being placed in the tooth with a temporary filling on top. Often this can be done in just a few minutes .This gets rid of the pain in most cases; the final stage of properly shaping the inside of the canals and placing the root filling is carried out at a later visit ( in your case in a few weeks , after your baby is born)
Corsodyl won't help a periapical abscess, nor will xtlitol.
Hope this helps.
Looking forward to hearing your news in the birth announcements thread sometime soon