I was on cetirazine 10mg when I was TTC. I reduced it to 5mg a day just before and during when I believed I would be fertile to increase CM. Cetirazine and other antihistamines can reduce CM. 5mg was enough to provide cover against my itch's for a few days.
I went back up to 10mg until I got my BFP.
We conceived first month of trying, so reducing to 5mg was enough for us to increase CM to a suitable lvl.
When you conceive you need to swap to piriton (Chlorphenamine) which is shorter acting and an old style antihistamine so can cause drowsiness. And you should get this via prescription and under medical supervision. I went docs and got it changed within a few days of BFP.
Dose for piriton :
Adults (including the elderly) and children aged over 12 years: Take one tablet every 4 to 6 hours. Do not take more than 6 tablets in 24 hours.
You could probably swap to piriton when you start TTC, but I have found isn't as good. You either have to take it more often and risk drowsiness, or don't have enough cover all day as one tablet a day doesn't last long enough.