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Managed to get a not very good photo of the guppies this morning. You can distinctly see the dark spot at the back of the abdomen. They have had this for quite a long time but no young have come along it

Guppys are always pregnant. YES she is pregnant but could terminate the babies due to stress.

found this:
Take care not to move the pregnant female too near her due date as the stress can cause her to absorb the developing babies or to give birth prematurely.

and this:
found that if I move the female too early she will abort the pregnancy due to stress,

Females are always in different stages of pregnancy, but do require a good male who is fertile to do his thing. It is not unheard of to have infertile males and a guppy who just doesn't deliver is most likely aborting internally due to stress. Your photo is hard to tell but yes, she does look to be pregnant, although only slightly. That dark gravid area will get bigger and darker the nearer she gets to delivery. Depending upon how young she is, it could be she has delivered but only a few dozen or so babies that got immedietely eaten. I have seen in guppy tanks a female deliver her baby right into the waiting hungry mouths of other guppies right behind her.

One thing to look for, That dark gravid area starts off like a light brown or beige area and begins to get darker at the back of her, like the sliver of your fingernail. As pregnancy progresses, the area grows more like the size of a small whole finger nail and gets darker, almost black looking. It's been a while, but you will be able to gauge once you pay attention to a few pregnancys and deliveries too. Quite possibly she delivered and you missed it and the babies were eaten rather quickly. Based upon that photo its hard to see but she looks early into pregnancy, not late term.
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The gestation period of a guppy is 22-30 days. The average gestation period is 28 days. After the female guppy is inseminated, a dark area near the anus, known as the gravid spot, will enlarge and darken. Guppies prefer water temperatures of about 28 °C (82 °F) for reproduction. After giving birth, the female is ready for conception within a few hours. Guppies are known to exhibit superfetation, that is, the maintenance of batches of embyros at various states of development. The female guppy has drops of between 2-200 fry, normally about 30 to 60.
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Once a female Guppy is pregnant, she can release babies about once a month for up to six months without a male in the aquarium. So your Guppy females may continue to release babies, even if you don't have a male Guppy now.

Here's a nice video of a female giving birth. You can see what the "gravid" spot will look like when she's ready and fully pregnant.

The belly will be swollen, the gravid spot large and dark.
good video showing close up of live birth:
http://www.youtube.com/watch?v=kFkBjD2Le3k

That was a really good video. I shall have to observe my guppies tomorrow

Well Penn, are you a daddy yet? Did that guppy finally give it up and have little guppy babies yet?