Here is a concept shot for Mistress Sarah. This happens 8 months after her first series. Sarah and Diane happen to share the same menstrual cycle and Penny spent the night with her after filling her mom with her thick creamy goo. They both got pregnant on the same day and they have the same due date.
Since they are so close to delivering, they can't accommodate Penny's enormous size anymore and Sarah brings in a surrogate to satisfy Penny's needs.