Hi. Greetings. I usually don't prescribing any treatment in particular during pregnancy . just use a physical sunscreen like sun stop 19. Avoid excess sun exposure. I am not endorsing this but some apparently claim application of besan - gram flour with curd apparently helps in reduction of tan. In any case see a dermatologist with your baby after delivery and then he'll plan your detan treatment.
Hope this helps. Warm regards. Dr Govind S Mittal.