I certainly wouldn't tell them as singleton pregnancies are pretty much all the same whether conceived by IVF or not. It was not on the claim form when I submitted it in March this year. However, if you had confirmed that you had a multiple pregnancy, you were asked if it was from fertility treatment.
What you actually need is to see the policy wording itself. If I can find my policy, I will look it up for you because sometimes the people that you speak to may not actually be right. Like they might cover single pregnancies, but not multiples. If you just say nothing, there would still be a risk if they changed their form to ask about singleton pregnancies from IVF that they wouldn't pay out. The question about multiples is on the form that the doctor has to sign for you to make a claim.