Maternity allowance for self employed

Hi all,

I’m not pregnant but I like to plan ahead.... I’m self employed but probably won’t earn enough to need to pay class 2 NI payments, but plan to pay voluntary. HMRC told me that to do this I simply need to tick a box that I want to pay voluntarily when I do my self assessment. Does anyone have any experience with this? How does it work if I want to claim maternity allowance but fall pregnant before I’ve paid my national insurance contributions?

Also, the HMRC website confuses me. Initially it says you could get £148.68 a week maternity allowance or 90% of your weekly average earnings (whichever is less) for 39 weeks.
But then it later says to get the full amount of maternity allowance you must have paid class 2 national insurance for at least 13 of the 66 weeks before your baby’s due. So does this mean that if I pay class 2 NI I will definitely get the full amount of £148.68, regardless of my earnings?

Thank you xx