Alyson Hannigan is pregnant, news that (as the link implies) makes it very likely that her character will become pregnant too. Her character is at the point where it would be plausible and a good source of story ideas for her to have a baby.
There are two options regarding a TV actor’s pregnancy: write the pregnancy into the show, or try to hide it. Both have their problems. When you hide it, you get those Seinfeld episodes where Julia Louis-Dreyfus spent all her scenes behind the counter in Jerry’s apartment, or those Cheers shows with Shelley Long behind the bar for 23 minutes. But unless a character has already been set up as wanting to have a baby, writing the pregnancy into the show often feels awkward and ridiculous — like Cybill Shepherd’s pregnancy on Moonlighting, or various pregnancies on Friends. CSI: NY is doing a whole elaborate pregnancy story that may or may not turn out something like that. And whether or not the pregnancy scenario is plausible, something worse is always on its way: once she has the baby, the show is stuck with that baby for a long time (unless they want to engage in SORAS, and that’s no longer fashionable in prime time). The child is always forgotten as the character gets back to doing what she was doing before, and we wonder why she’s not spending any time with her kid.
See here for more examples of real-life pregnancies and how they’re dealt with in the TV world.