To both DolphinMan and Guppylover,
Basically, you can't tell how long such a female seems pergnant. For it's very individual how a pregnant female's belly will grow. And the other female can drop fry when she's not that big at all and another female will drop after she's really, really huge... So, it's hard to tell. And this answer is coming from me. And I'm a keeper and breeder of guppies and other livebearers for over 40 years now.
When a female guppy has delivered fry, you can't tell if she gets pregnant right away. Also this is an individual given. For a female livebearers like e.g. guppies, swordtails, mollies and platies will decide themselves wether a pregnancy starts. Just like Brianna (Lilme) already mentioned that a female can store sperm. Btw, it's even a bit longer than 12 months that she's able store it. So, a female doesn't have to get pregnant immediately by all means...