Summary: As discussed in a previous post, saying that tzadeka mimeni are two separate statements ("she is righteous; the pregnancy is from me"; or else "the situation is from Me") is at odds with the trup, which has no pause between them. Yet Rashi endorses this as both peshat and midrash. And it appears that so does Ibn Ezra. Here, I give further thought to what could influence this explanation -- the word yakir and the otherwise absence of an admission by Yehuda.
Post: As discussed in the summary, saying that tzadeka mimeni comprises two separate statement appears to go against trup, but is endorsed by both Rashi and perhaps Ibn Ezra. In giving more thought to what could influence this interpretation, I had two ideas.
First off, in the pasuk:
what does vayaker mean? Does it mean that he recognized these signs? Or does it mean that he acknowledged and admitted them? The first is a private recognition, while the second is an overt admission of guilt and that he was the one who had slept with her (or at least could be).
Above, JPS renders it "acknowledged", but Judaica Press has instead "recognized":
Then Judah recognized [them], and he said, "She is right, [it is] from me, because I did not give her to my son Shelah." But he no longer continued to be intimate with her.
I think we see both meanings to this word. In this week's parsha, when they bring Yosef's ketonet pasim, Yaakov recognizes it. (Though he also identifies it as that of his son.) But in parshat Ki Teitzei, ki et habechor ben hasenua yakir, I think it means "acknowledge".
Once it is acknowledgement, we might expect the rest of the pasuk to be the text of the acknowledgement. He says that it is his fault, in not giving her Shela. That is an admission of sorts. But where does he overtly confess that he is the father? Now, I think as a matter of peshat, there was likely this admission of being the father, which is why she was not killed, and why she was tzadeka -- and not that he was saying that she had the right to sleep around because he did not give her Shelah. But the way Rashi reads the text, he explicitly admits this. And so it works out quite well in the narrative.
(As an aside: If this is so, perhaps the acknowledgement should be about the items, rather than the pregnancy.)