The victims are kept in a basement, well-fed and cared for only until they give birth. Immediately after delivery, the mother is murdered.
In a tense climax, JJ and Prentiss lead the charge into the Reinman home to rescue the surviving women and babies.
You can watch the full episode on platforms like Apple TV or Philo . [S5E5] Cradle to Grave
The "interesting story" you're referring to is likely the episode " Cradle to Grave " (Season 5, Episode 5), which is widely considered one of the series' most disturbing and memorable "unsub" (unknown subject) cases.
The couple is trying to "replace" their lost son. They keep the male babies that meet their specific criteria and discard the mothers and "imperfect" infants. Why It’s Notable The victims are kept in a basement, well-fed
The team initially suspects a black-market adoption ring. However, the profile shifts when they realize the killers are a married couple, Robert and Linda Reinman, who lost their own baby, Michael, due to a medical tragedy.
Subplot-wise, this episode features Chief Hotchner being unusually hard on Derek Morgan, a move later revealed to be a test to see if Morgan is ready to lead the team. You can watch the full episode on platforms
S5E5 - “Cradle to Grave” - Unsubs: A Criminal Minds Podcast