Advice on when to let my job know I'm leaving?

I've made up my mind that I'm not coming back to my job once I go on maternity leave. I'm still unsure about the timing because I don't think I'll physically be able to work very long into the summer. I'd be shocked if I make it to July because I've had a rough pregnancy so far and I likely have preeclampsia now. For that reason, I feel that I should let my job know beforehand. Like as in fairly soon. I work at a popular vet clinic and they have a horrible habit of not hiring replacements until the very last second. Like not even interviewing people until the former persons last days. I'm hoping to avoid that because our summer months are UNBELIEVABLY busy and it takes a looong time to get trained and used to working reception there. My only concern is that they'll treat me differently until I can't work anymore or that they'll want me to leave before I'm ready to financially. So I'm not sure what to do.