Season 7, Episode 14: "Power Play"
A has done some pretty horrible things over the years, but they're all outdone here. Emily went through IVF to donate her eggs to earn money. Alison is pregnant, and discovers something extremely troubling — she's carrying Emily's baby, after being drugged and nonconsensually inseminated with Emily' eggs while at Welby State Psychiatric, a mental health facility.
Besides this huge realization, we find out something about Pastor Ted Wilson (Edward Kerr), who is dating Hanna's mother, Ashley (Laura Leighton). Pastor Ted is Charlotte's father (alias CeCe Drake and born Charles). Charlotte is the biological cousin of Alison and her brother Jason (Drew Van Acker). Alison's mom, Jessica DiLaurentis (Andrea Parker), told her twin sister Mary Drake (also played by Parker) that Drake's daughter Charlotte was dead, even though she wasn't. As a result, Drake seeks revenge on Alison.