I used to be a HR manager and dealt mainly with sick absences
You really need to clarify exactly what the warning was for before contacting anyone else, also check your companies sick leave, maternity leave rules and disciplinary procedures so you know what you are due and wether they followed the correct procedures in regard to your warning, as stated before if the warning is including your pregnancy related absence then you must put in an appeal against this.
contact the Equal ops commission as this is what they deal with and specialise in, although the CAB are excellant they deal with everything and the equal ops commission will be able to give you specialist advice.