Just Removed IUD

My boyfriend and I are looking to try for baby #2 so on Tuesday morning I had my Mirena, which I had in for almost 5 years, removed. The OBGYN office I went to was new so the visit was quick and kind of impersonal. After removing the IUD the Doctor told me to wait until after my first period to try and start to conceive. She didn’t say why and just walked out of the room.

I read online that Doctors say that to make it easier to calculate a conception date. I’ve heard it can take months to get the first period after IUD removal so if we tried before waiting on a period would it make figuring out a conception date that hard?