'Derrick, this is Marla,' Hebe explained. In front of his eyes was a naked woman, probably in her early 30s, and he had to admit, even to a baby like him, she was very pretty. 'In a few minutes, you're gonna be inside her! She always wanted to be a mommy, and now thanks to you, she can!'
Hebe pulled the umbilical cord out of the pool and pushed it into Marla's pussy, and he felt it connect with the insides of her body. It felt weird, like suddenly he was part of her. Marla then sat on the table where Derrick had been and spread her legs widely. Hebe helped spread her pussy, and by now Derrick's body was so shrunken that she was able to push his feet inside Marla. To his surprise, it was indescribably pleasant and warm inside. With a little more help from Hebe, his body slid inside Marla, and he suddenly developed consciousness of things outside what his eyes could perceive (which was pretty much nothing). He saw his form pretty much from the third person, and could see Marla's body outside. He watched in this weird perspective as a large bump formed in Marla's belly as she became pregnant.
Derrick didn't get to enjoy this state for long, as his body rapidly regressed by about a week every few seconds. His form de-developed, his arms and legs receding to small nubs, he felt his genitalia shrinking into nothingness, his body shape changed from humanoid to a cluster of cells, and before he knew it he was nothing more than a fertilized xygote. A moment later, he felt something pulled from himself- the sperm which had made him who he was had detached from him, and left him- them, they supposed- an ovum lodged in Marla's womb waiting to be fertilized.
They kinda hoped that the body they would have when they were born again would be Derrick's, but they were already kinda detaching from the concept of him being a part of who they were.