Put a male guppy in with a female guppy and chances are good that within a very few days you will have a pregnant guppy. A female guppy can become pregnant about once a month. Given the fact that the average gestation period runs is 28 days, sometimes a bit longer, and sometimes not more than 20 days, the female is quite apt to spend most of her life being pregnant. Put a male guppy in with several females and within a few months you could have hundreds of guppies and a definite need for more space to keep them in.