Starting bc for IVF

Hi! I’m getting ready for IVF in November. Anyone that could help me understand this process it would be greatly appreciated. I started bc today. And I take it until October 25th and then I got for an ultrasound the 31st, I also start stims on this day. And then egg retrieval November 10th or 11th. My question is, is between the 25th and 31st is my period supposed to start or hold off? They’ve told me to take a 21 day supply and skip the placebos and go straight into the next pack and just stop those on the 25th. The way I understood them I’m not supposed to get a period. But I’m not sure.