Guppies are extremely prolific livebearers that incubate their fertilized eggs inside their bodies. "How long is guppy incubation?" is almost a trick question. It has two answers. One is "30 days," and the other is "the rest of the female's life."
Guppies are famous for breeding freely and rapidly, so it may surprise you that many females mate only once in their lives. It'll probably surprise you even more to learn that this does not limit the number of their pregnancies. While some females breed many times with many males, others store the sperm from a single romantic evening for the rest of their lives, and this one liaison produces new babies every single month.
Guppy Incubation and Birth
Once mated for the first time at approximately 3 months of age, guppies give birth about every 30 days. The mother guppy fertilizes a new batch of eggs each month, either from fresh sperm from recent encounters, or from older sperm stored inside her body. As soon as she gives birth to one batch of babies, she becomes pregnant with the next. These babies hatch live from her body carrying the remains of their yolk sacs, which feed them for several days until they are large enough to eat plants and detritus in their tank.
In a word, there is none. After the mother gives birth, her job is done. The young fish -- called "fry" -- are on their own. These little wiggling specks must immediately hide from adult guppies, including their mother, to survive. "Small swimming thing" translates into "food" in fish language, and the mother guppy does not have a very long memory when it comes to recognizing her own offspring.
Caring for Baby Guppies
Guppy keepers have a variety of strategies for protecting baby guppies from adult guppies and other fish. One is to have a separate tank dedicated to raising the babies. The gravid (pregnant with fertile eggs) mother is placed in the special tank when she starts to get fat, then removed right after giving birth. Another strategy is to simply have a heavily planted fish tank with lots of hiding places for new fry. A tank with ample cover and live plants will allow about 10 percent of the babies to reach adulthood, or about three or four out of every batch. Consider your ultimate goal in keeping guppies when you choose a strategy. A dedicated tank will allow you to raise several hundred young every year. If you are going into the guppy business, this would be the choice for you. If simply having one decorative fish tank is more your speed, letting nature take its course will be a better option. This is not meant to be cold-hearted: After all, a 10 percent survival rate allows you plenty of new fish to replace your originals as they age and pass away, while keeping the population at a sustainable level. It is also comparable, or better than, survival rates in the wild.