20 pills, one missing! Changing brand! Helpp

Lauren
Hi guys! 
I've lost 1 pill within my packet that i am about to start taking, i am going to get some within the next week. If i miss 1 pill will i still get a bleed at the end of the packet? & any chance of pregnancy if missed? 
Also, i have been taking the rigevidon combined pill, if i swap to microgynon half way through, will the cycle be messed up? Or is it better to stick to one brand?  
Thank youuu :)