Source: Biology textbook
Considering both Skitties and Wailords are mammals, the embryo should develop in the body… But if the Wailord is actually a fish, which fertilises the eggs by releasing sperms into the water and just wait for them to fertilise the eggs, possibly the reproduction is actually artificial and the waiting-for-the-egg time is the time needed for the day-care workers to transplant the male and female gametes(I.e. The sex cells) into the egg shell.