They breed like rabbits. More often than not, an Eastern female will become gravid after even one "session".
Things which can affect the % rate of success:
The age/size of the female, body condition and how it was cycled for breeding.
As for phantom pregnancies , I've had small and skinny females have small litters. I've had heavily mated, well seasoned mothers in very good body condition not produce a thing.
I found the easiest way to keep track on matings was to house blueys singularly and only pair them for the brief act of mating. A keen male who knows what needs to be done will have the deed over in 10mins. Females need to play their part too, if they aren't in season they may go the biff or just will keep running whenever the male comes near. Watch their body language , you'll learn the patterns of what to look for.
Here's a vid of some Northerns getting cranked in my blue tongue rape dungeon. Easterns are much the same.
[video=youtube;hTxW_eCnf7w]https://www.youtube.com/watch?v=hTxW_eCnf7w[/video]
And another vid, the forum won't let me me embed 2 youtube clips.
https://www.youtube.com/watch?v=1v3ykl8-pug
Hope this helps.
Cheers,
Dave.