From Longman Dictionary of Contemporary Englishmonth by monthmonth by monthSLOWused when you are talking about a situation that develops slowly and steadily over a period of time Unemployment figures are rising month by month. → month
Examples from the Corpus
month by month• Unemployment figures are rising month by month.• Because you produced it on loose pages I could exhibit it month by month as you organised it.• Let your astrologers come forward, those stargazers who make predictions month by month.• The hon. Gentleman has no evidence for asserting that standards have dropped month by month.• The market is changing not year by year, but month by month. 