Behind the cut are spoilers for the rest of Season Five.
So assuming that the NCIS PTB didn't just screw up and they do know there medical facts (thanks to meredith44 for all the information pertaining to this), and/or they don't think that Ducky and/or Abby are stupid, then . . . the blood almost certainly cannot be Jenny's. Why? Because according to one of the links Meredith very kindly provided me with, 'the results for a CBC vary based on whether it is a female or a male'.
Thus Ducky 'sticking his neck out for Jenny, isn't 'for Jenny' insofar as it's her tests (well blood at least), but 'for Jenny' insofar as she's asked him to run tests on her behalf.
Let us assume the writers haven't screwed up (and to be honest with their previous records at times I'm practically 50:50 on that) and they don't believe either, or both, Ducky and Abby are stupid then the blood is from a man. What man could it be?
1. Jenny's father (toomuchfandom must muted this idea on one of my previous speculations, going on to say that Jenny is looking after her sick Father and not really out of the country at all, as Ducky vanished at least once when Abby gave him the blood test results). Okay, this is possible, La Grenouille is adamant he didn't kill her father but that he took a bribe from him. Jenny is adamant her father is dead and the military information supports this (it says suicide), but we also have Colonel-General Dimitri Borov who swears her father visited him.
2. A man purporting to be Jenny's father. Because of the above, Jenny wants to be sure; she knew her father was suffering from some disease there are several illnesses/diseases elevated CPK levels can show.
3. Jenny's son. And ::gulp:: Gibbs is the father. My reasons for this idea come mainly from SpoilerFix.com's comments pertaining to the final double episode: 'We've had flashbacks to a relationship that Jenny and Gibbs had in Paris while they were working undercover,"'says executive producer Shane Brennan. 'Now the secret past will come back to haunt them, leading to an emotionally devastating two-hour season finale'.
Okay, so what happened in Paris?
a. Jenny killed a man (deliberately or accidentally?) That is something that has never been resolved and it could still tie with with La Grenouille in some way.
b. Gibbs was arrested and Jenny stood across the street watching and smiling in a self-satisfied way. Again, this was another thing that was unresolved.
c. Gibbs and Jenny spent a considerable time in bed. Now this had never sat well with me, and not because I'm a Gibbs/Ducky shipper (it's never sat well with J or my parents of other non-shipping friends/contacts), but because it is SO OOC for Gibbs. For him to screw around whilst on a case, has never been believable to me. Unless, of course the undercover operation involved them being a couple so all the bedroom scenes and flirting, etc. etc. was an act. Act or not, Jenny could have got pregnant, even assuming (which I am) that one or both of them took precautions, unless one or both of them had been sterilised (and we know there are even cases there of it somehow going wrong) pregnancy could have resulted.
So it did. Jenny had a son, but didn't want Gibbs in anyway involved. At least one of the things for which the CPK test is carried out is for something that most commonly occurs in children age 5 to 15 and adults age 40 - 60.
I sincerely hope that, assuming TPTB haven't screwed up, etc. the male being tested is most definitely NOT (c), but (a) or (b). The first two would make a lot more sense to be honest, because of the whole 'is he alive/isn't he'; now whether it is her father or not, I have no idea.
OTOH, I can see Ducky being more prepared to go out on a limb if a child is concerned rather than an adult.
But then again (I argue with myself *g*) he was concerned by Jenny's state of mind and her obsession with La Grenouille, not just for her but for the effect it was having on the team in general, the risk she was putting team members under. Thus, if she came to him with the whole thing about 'a man who claims to be my father', actually I could see him being prepared to help her, for the state of her mind/the rest of the team.
I know several people have expressed surprise that Ducky would go out on a limb for Jenny. I myself have always got the impression that Ducky wasn't overly fond of Jenny, his manner with her seemed more reserved and slightly aloof compared to how he was with the rest of the team. I know she's now Director, but they worked with one another when she was just one of Gibbs's field agents. But if he helped her with this and maybe helped her get to the bottom or her obsession, then it would benefit the rest of the team.
OTOH (how many do I have?) the 'We've had flashbacks to a relationship that Jenny and Gibbs had in Paris while they were working undercover. Now the secret past will come back to haunt them, leading to an emotionally devastating two-hour season finale', does rather worryingly maybe point to a child, because it specifically mentions the 'relationship' and 'the secret past coming back to haunt them'.
Unless, as I've written in more than one story, Gibbs knew Jenny killed the man deliberately (as to why, who knows, again I've written it a couple of ways) and covered for her because of their affair.
So yes, I want the blood to be her father/a man purporting to be her father but . . . I am now having this sinking feeling it's going to be a son. And if so, then it will be Jenny leaving and Gibbs is left 'holding the baby' so to speak.
One final glimmer of hope is what I said to aingeal8c yesterday, Jenny already knew she was pregnant, maybe the father was dead (heck maybe the father was La Grenouille or the man she killed) and so kept sleeping with Gibbs so she could, if she ever needed to, pass the child off as his. (Okay, that's more wishful thinking than anything, I guess. I don't think Jenny is that conniving).