Babies and Traits

[Execution]

Throughout the pregnancy, I've made sure to keep Mom's mood & needs as high as possible. When the baby came (he was delivered at the hospital), he was born with the Artistic trait (I wasn't able to choose this) and a trait I was able to choose. Moments after he was born, my game crashed before I could save. When I re-played the birth, same result; Artistic Boy with a selectable trait. I know that gender is locked in place at Day 2 (according to Carl's Guide), but what about traits? Was the fact that he got Artistic both times a coincidence or are babies locked in place with traits as well?

From my understanding, outstanding mood & needs = 2 selectable traits. Good pregnancy (but not top-notch) = 1 good trait & 1 selectable trait. Okay pregnancy = 1 good trait, 1 okay trait. Mediocre Pregnancy = 2 okay traits. Poor pregnancy = 1 bad trait, 1 okay trait. Horrible pregnancy = 2 bad traits. Is this true?

My hypothesis now is that the baby's possible traits are locked in place at Day 2. Based on the above paragraph, and example would be; Baby X could get 2 Selectable traits, the lucky trait and a selectable trait, the lucky trait and the excitable trait, the excitable trait and the absent minded trait, the absent minded trait and the can't stand art trait, or the can't stand art trait and the unlucky trait. Baby X cannot get the Absent Minded trait and the Virtuoso trait in any circumstance (if both traits are unselectable).

Also; I've noticed traits can be inherited from ancestors several generations back. I've had a child (in Generation 3) that assigned the Absent-Minded and Excitable traits at birth. My founder had these two traits (but no one else that I know of).
My current baby's (the Artistic one I mentioned earlier) unselectable trait isn't found in anyone I am aware of in the entire family tree, however.

My final question; how are the traits unselectable traits selected? Are they picked at Day 2, are they genetic, or are they picked at random?

[Sophie98]

So maybe your pregancy was good but not quite good enough to let you select two traits. I always save after having a nooboo so I can't help you with the second problem. I think all traits are probably at random, and some are inherited or selected by you.

[Pam]

@Execution:

You're putting way too much thought into babies' traits.  You get to select one or two traits based on how well the pregnancy went.  The traits that you do not select are picked randomly by the game.  It doesn't recognize traits as being good or bad.  They're just traits.  They aren't inherited, either.  If they show up more than once in a family, it's just coincidence unless the player selected it.  I suspect it was also coincidence that your baby was born with the Artistic trait both times.  I've had to replay births several times and the traits come up different if I'm not choosing them.  You've got a great theory, but I really think it's much more random than what you're saying.

[BellaClo]

Like Pam said, assigned traits are completely random. There is only a limited list of traits available for babies. You can look at this table here on the guide. 23 for the base game and 25 with all EP's.
 
You see them more often because Sims born in game always have 2 traits coming from this list, whether they were assigned by you, or picked up by the game. This can lead to an impression of genetic or inheritance, but it's just that, an impression. The only genetical aspect in the game is related to the look of the Sims, and it's quite a superficial system.