Hi, as you have mentioned the dark marks are there 18 months( were there before you were pregnant). this is postinflamatory hyperpigmentation
Zimig cream is anti fungal agent , now you can stop using it as there is only pgmentation
You can take following treatment:
hydroquinone2% cream mixed with hydrocortisone
sunscreen 25+ during the day.
tca spot peels
oral beta carotete and topical retinoids
should be avoided till you are nursing the baby, start later.
contact a dermatologist
for further evaluation of the cause and treatment if this doesnot help.
hope this adresses your concern