What's the correct translation for 1.5 months? Is it "一个月半" or "一个半月“? What's the correct translation for 1.5 months?

Is it "一个月半" or "一个半月“?
Technically 一个半月 is better and more common in written scripts. Although I don't understand why 一个半月 returns few results in Google, but those mentioning 一个月半 seem to be mostly related to infant raising. I'd say both is okay if you want to use in informal speeches.