Worth buying or too difficult/inaccurate? They're supposed to work on mares over 120 days - delivery. Any other suggestions for tests? Vet can't palpate and I don't have unlimited funds (found an external ultrasound machine I wouldn't mind having for 2000$!). I have 4 horses to test at various dates so don't really want something you have to use within a very specific time frame. Any help appreciated!