Guppies are relatively easy separate by gender - males are slimmer, with larger tails and more colour - females are larger - with less colour and usually have a smaller tail.

Female Guppy:
A pregnant female Guppy has a dark spot on the rear part of her abdomen. This spot is called a gravid (grav-ed) spot. they also have triangular shaped pelvic fin just behind the gravid spot.

To see a guppy giving birth click here, courtesy of fishforever.co.uk 
*you will need windows media player to see the movie

Male Guppy:
A male Guppy has a modified fin called a gonopodium (gone-o-pod-i-um). This fin is used to fertilize the female guppy - all male live-bearing tooth carps have a gonopodium. Male guppies are usally smaller.