Who are (little) Ren and Satsuki's parents? I have read a few theories online.
Theory one:
Ren's parents are Takumi and Hachi.
Satsuki's parents are Nana O. and Ren Honjo.
https://shojocorner.wordpress.com/2012/02/01/nana-baby-momma-drama-spoilers/
https://nana-confessions.tumblr.com/post/55188511920/i-have-no-doubt-that-ren-ichinose-is-takumi-and
Theory two:
Ren's parents are Nobu and Hachi.
Satsuki's parents are Nana O. and Ren Honjo.
https://komatsunana.tumblr.com/post/140001487839/ren-and-satsuki-who-are-their-parents
My theory:
Ren's parents are Reira and Takumi.
Satsuki's parents are Hachi and Takumi.
I think this would explain the reason for the distance between Takumi and Hachi. The dampers on this theory would be a dialogue where Hachi talks to Satsuki about her big brother, most likely referring to Ren, which implies that he is older than her even though Hachi was the first person to become pregnant. The other damper would be little Ren calling Reira Reira, instead of Mommy when he calls Takumi Daddy. Reira also wonders aloud why she doesn't hate little Ren.
I'm pretty sure they're both Takumi's (unless Ren is actually Nobu's but I think that's unlikely).
Since Hachi called Ren Satsuki's older brother, he's obviously older. Even if Nana O. or Reira did get pregnant, the child would have to be younger than Nana's oldest child. Ren also happens to look a lot like Hachi. Satsuki looks a lot like Takumi so I'm 99% sure that's his kid too.
Considering how long Ren and Nana O. hadn't seen one another, I'm pretty sure its impossible for her to have found out that she was pregnant after his death (since it would have been quite some time), especially since she's on the pill. In addition Nana O. also said that the pill (is not necessary) signifying that it had been some time since they had last slept together. Coke also causes fertility issues, and Ren being on it was a big part of the series.