Well, it depends on which child you're referring to. There are two children that are referred to in the glimpses of the future that we get in the manga: Satsuki and Ren. Satsuki is the little girl and Ren is the boy.
The father of Nana's child is not currently positive. However, it is either Nobu or Takumi. Since Nobu used condoms, it is widely believed that the child Hachi (Nana) is pregnant with belongs to Takumi. However, we can not be sure until the child is born. Satsuki tends to look like Takumi in the glimpses we receive of the future, so most believe that he is the father. Also, he does not use condoms and Nana was not on Birth Control at the time of her pregnancy. Due to these factors, chances are that the child will be Takumi's.
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Takumi....... When Nana Found out she had been going out with Nobu for a week or two. and she was six weeks along.
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Manga because not only does it explain a lot more, it has a much better ending, before the hiatus that is.
There are 47 episodes .and there is a rummer that they will make a second season after the manga ends
Nana to Kaoru
I think its Charmless Man - Blur
it has a really crappy ending, nana is now engaged to ren, and hachi(the other nana) is married. Right before they go on tour, Shin is arrested for procession of marijuana which caused blast to withdraw from tour. Nana begs ren to take shin's place but ren refuses. Nana decides to go on with a solo career, promising to come back and reform blast with more fans than ever. then 6 years later, eveveryone meets back at apt 707 except for nana, who appearently left the country without telling anyone them. hachi have a 6 year old daughter, and they were at the apt to watch fireworks. Hachi received pictures from a stranger, pictures of Nana with long blond hair at what appears to be england. Hachi cries, and hopes that nana will reunite with ren someday. manga contiues.. there's no second season of nana YET!