Average earnings for Mat pay

I know your maternity pay (the 90% of your salary for 6 weeks) is calculated by taking your average earnings across a particular period during your pregnancy, does anyone know if there’s a set time that this period is?

I’m currently signed off work due to an injury, I got two weeks full pay for sickness but am now on statutory sick pay, and I’m part time so earning around £50 a week. I’m worried if I’m off for much longer, these earnings are going to be used to work out my average earnings, then when I get my maternity pay I’ll be coming away with much less than I’d normally have got.

I get if I’m not there doing my job, I can’t expect to be paid the same, but my work pays enhanced mat pay at 100% of my salary for 12 weeks so I’ll be missing out on quite a lot of money if I’m not back at work before the period of average earnings begins.