Irregular periods

My period used to be regular as clockwork but since we've started trying for another child they keep getting further and further apart (last month it was 40 days between periods) We've been trying nearly a year now and it's really hard to know when to take a test or not now cuz I don't know if I'm actually late or not. I'm on day 37 at the mo this month :( has this happened to anyone else? How do you know when to take a test if your period hasn't come ? Xxx