every female guppy has a gravid spot some visible some not, it may be that your guppies are young or not pregnant,
i wouldnt worrie 2 much as you would notice if they were distressed,
pictures would help
i have noticed my guppy fry get a gravid spot about 5 weeks when they atert to colour up this is when i seperat the females as males become sexualy active extremely young,were as the females not to so young, if they are young guppies they may have given birth to 2 or 3 fry as they dont have many first time round if this is so they are likley to have eaten them, the older the guppy the more fry it will produce,dont expect to much to soon,
personally dont worry they will come to in their own time
hope this helped
