You can conceive as soon as you ovulate. I think usually they recommend waiting a cycle for dating purposes. I’ve always tried after waiting 2 weeks or so on the basis that I will only get pregnant if my body is ready to.
I fell pregnant after my first period back from a miscarriage I had September last year but my periods still weren’t regular and hadn’t been before my miscarriage
I had a miscarriage in April and ended up pregnant in June. Never had a period though.
I was advised to wait until after my first cycle. I had a miscarriage in January and got pregnant in March. I’m due on Christmas Eve.