Anyone work for the nhs help with mat pay calc?

I’ve gone on my maternity calculator at work, and you have two months where you have “Average weekly earnings” which make a difference to your Mat pay. I had a baby which was due June last year and I remember my months were Dec and Jan. I’m due a baby next May and I’ve used a couple of calculators which look the same but give me different months each time, (either Dec and Jan or Nov and Dec) so I would have thought it’d be Nov and Dec for May… has anyone else had this problem? Or does anyone know which is right? Thank you!