I am just curious

I am in a committed relationship and i have been trying to get pregnant for four months now. and my test were negative. here's the conundrum. i am very irregular and my cycle only comes like three times a year. i took birth control for about three days and got a period. the question is, if i do that again how long should i wait to have sex so that window stays open for a baby. my first child i conceived while i was on depo. i see that she wanted to come into this world at all cost.
if i do take bc for a couple of days, will my chances of being a mom for the second time be greater than trying for months straight, no skipped days. my fiancee is getting deployed soon and i want to give him a second child. i'm lost.......help'