My Molly is due today. All is well, but she shows no signs of labor. Her temperature was 100.0-100.5 over the past week and has been averaging about 99.3-99.5 for the past two days. I've been eagerly awaiting a more drastic drop

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Here's my real question. Molly and daddy-to-be are both mine and live together. The time span between the FIRST time they connected to the LAST time they connected was
NINE DAYS. Most due date predictors say to use the FIRST mating date as a reference, which makes her due date today.
My question is, is it possible that I have calculated her due date as being too early? I don't want to worry, but I also don't want to allow her to be too overdue if she doesn't deliver tonight (all the signs are indicating she most likely wont).
At what point would most of you consider this litter to be overdue and bring her to the vet?