There are two reasons why a hypno is lurking about your property: one good and one bad. The bad possibility involves the fact that hypno feed on sweet dreams, particularly those of small children (as they have particularly potent imaginations). It’s possible that this hypno has sensed the fact that you’re expecting and is thus waiting for a possible meal—or feeding on your hopeful dreams.
The good possibility involves the fact that hypno are psychics. They can sense negative energy, even if humans can’t sense it themselves. These hypno, then, are not there to feed off you or your child but rather to feed off whatever threat it perceives around you in order to protect your dreams. (Why it would do that tends to vary from hypno to hypno. Some do it to preserve a possible food source. Others do it simply because they revere small children due to their powerful imaginations.)
In order to determine the cause of your hypno’s presence, start off by having your houndoom use Odor Sleuth to ensure there are no negative energies in the house. If there aren’t, approach the hypno as cautiously as possible. If the hypno attacks you, drive it back with houndoom. It may then be a good idea to invest in a guard ghost, such as a mismagius or banette, to drive away the hypno with negative energy to counter your positive. Partnering your baby with a munna or musharna (which can “claim” your baby and thus discourage the hypno) might also help drive hypno away.
If, however, the hypno doesn’t attack you but instead continues to watch your house or something around you, ask it to join you or try to catch it. Even if it’s malevolent, catching it (when it isn’t trying to attack you) gives you an advantage: you have the ability to train the hypno to leave your child alone or store it in the PC system to keep it away from your child. If it’s benevolent, then asking it to join you will bring its protection into your home. It’d still be a good idea to train it to leave your child alone, however, as it’s still a pokémon that feeds primarily on dreams and thus needs to be taught to go against its instincts around a potent source of them.
As always, best of luck, anonymous!