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Any mid sized /aged female Guppy, housed with a male Guppy will be pregnant. It is just the way of the Guppy world. They can store sperm from the males for 6 months or more...after being separated from any male and can continue to have birth cycles.
You will know if it is close by her looking square and kinda shiny, and I mean square! Sometimes a young/new to birthing momma guppy will have few babies the first round, and increase in number as time goes by.