Halfway trying?

The only contraceptive that my husband & I are using is the pull- out method. We're giving the control to God when He's ready for us to have babies but my question is, how likely is it that I'll get pregnant with him pulling out every time? 
​My period is VERY irregular (I'm 3 weeks late now & not pregnant) & I've always been really concerned if I can even get pregnant with such an irregular period. I'm made to be a mom & not being a mom is my biggest fear. Any advice?