Genetics - When in pregnancy are physicals traits determined

[bewitched1618]

I was unsure where to put this one, but as it seems like it might help the pregnancy section of the guide I figured this was the best place. 

I've been doing some testing with a couple I have and I'm having trouble pin pointing when physical traits are determined.  I know about choosing girl/boy and number of babies.  What I'm curious about are actual physical traits passed down from the parents (nose, eyes, eye color, jaw, mouth ect.)  Each conception changes the baby's genes, but what was interesting is that if I save right before she goes into labor there are already some set traits (nose, hair color, eye color).  Which is cool, but a little annoying as I find playing those same four days over and over again tiresome (and time consuming). Does anyone who knows how to look at the actual code have any idea when the no-turning back point is?   Thanks!

[Trip]

From the (very limited) testing I've done, facial features and weight are set at birth. Coloring seems to be set...sometime before birth.

There was one dynasty attempt of mine over the summer where my heir was born and I aged him up to toddler, but the game crashed while I was in CAS. My last save was from a few hours before his birth. When I redid all of that, he had entirely different features, but the same unusual coloring (he got an odd shade of black hair that neither of his parents had both times). There was a similar situation in another game of mine; kid was born and aged up to toddler, bug forced me to go back to a few hours before her birth, and she went from having a blend of her parents' features to looking like a clone of her mother, but both times, she had the very specific combination of her father's tan skin and her grandfather's silvery-blond hair.

As for weight, I once had a sim who was very large in spite of having slim parents and grandparents. The culprit? Her father, who got abducted and impregnated by an alien and was quite fat at the time of her birth.

That's the extent of my testing. I always wanted to do something a little more formal just to confirm, but I don't believe anyone who says that genes are determined at conception. And I haven't looked at the game code for it.