
My best advice would be not to leave her longer than 2 days over her due date, counting from the first mating, even if conception didn't take place immediately. If she's in obvious distress prior to 2 days over, clearly you'd need to speak to your vet who will probably ask you to bring her in (or come out!). Sometimes with small litters, the puppies can just 'sit' growing bigger by the day which could mean a C.Section is more likely to be needed. or at least a shot to get things started. Be aware, sometimes a car trip can do the same .... have somebody go with you to watch your bitch, and take a box with a warm pad and your whelping gear with you. And be prepared to pull over! Athough I did it alone with my first litter, having a mentor, or having experienced birth yourself, can be a huge help.
Good luck!