If you're applying for the 2019-2020 cycle, then I don't see any reason why that would be an issue. If you're applying for the 2018-2019 cycle, then it depends how well the professor knows you up to this point. The professor will also be challenged to adequately assess your performance in the class since the semester isn't over yet.
However, as I said at first, if it's for the 2019-2020 cycle, I don't think that'd be an issue. I'd just let the professor know at the end of the semester that you'd like to ask him/her for a letter just so that he/she can remember who you are come May when letters can be submitted.