hefty gravid spot. thing about guppies are they give birth when they are ready. there is no real way of telling when. I've got a big female that i've put into a tank by herself to give birth to all her young so i could breed her with a specific male. it took nearly 6 months and she would give birth to 3 or 4 every few days. I've had others that give birth to 20 in a day and not give birth for another month even though they have a gravid spot like that. it's all just kinda a crap shoot.