OB care ?

If a patient has only 4-6 visits with us than transfers out to another doctor,,,or transfers to us after seeing another doctor. Is there a diagnosis code for transfer of care? Or,I saw a post from someone talking about modifier-TH "OB tx/services,prenatal or postpartum. Sometimes we get questioned from Ins companies about why a patient had a certain amount of visits even though we use the "antepartum codes". Would a modifier help?
Or any other ideas would be helpful. Thanks