I just experienced something weird in my game which had never happened before, at least not in my active family.
Not sure if there's something I changed somewhere, but I haven't played with my settings for a while aside from allowing the population to be a little higher, so I'm lost.
My active family has 10 Sims in it, which consists of mother, father, children and fiancé/spouse of children.
I just made one of my Sim travel to France for 8 days, and before she left, her sister Eliane was about 4 hours pregnant. My Sim came back before the 8 days were over, and when she arrived back home, there was a new sim in my active family, a child, who turned out to be the son of Eliane and her husband. And Eliane is still about 4 hours pregnant.
Not sure why this happened, but I don't really want it to happen.
Does anyone knows what settings I can change so this doesn't happen again?

I've had this happen once or twice, except that the sims were no longer pregnant. From what I understand, many worlds borrow the demographics of France somehow behind the scenes. Sometimes pregnancies will get resolved in our absence and, since there are no preggos, babies, or toddlers in France (by default anyway), the resolution takes the form of a child. In my most recent case I was able to explain the anomaly away because of the unusual way in which I was traveling in that I wasn't re-entering the world with the same sims who left it and there were other worlds I had played in between. Plus the pregnancy itself wasn't planned (by me) and had formed when the parents were actually invited guests in a different world and did Risky Woohoo when I took my eyes off of the for a few minutes, so by virtue of me sending them home they had to pass through the usual TravelDB process.

I know that strange sequence of events is not what you have described here, but I don't think there's a way to prevent it entirely 100% of the time from what others have reported (except for not traveling when you have a pregnant sim at home). I would probably keep the child but force them to age back down to toddler at the risk of some glitching due to CAS not being used the way it was intended, and terminate the persisting pregnancy. Or of course you could TA the child who was never supposed to have been formed and allow the pregnancy to resume if it's still viable.

Thank you for your explanation, igazor.
Unfortunately, I have no ways of explaining why it happened on my end.
I always believed that the howeworld was in a "frozen" state while traveling and since that pregnant Sim wasn't part of the trip, it never crossed my mind that a new kid would be there when my adventurer Sim came back.
I already TA'd the kid, since he was already too old for what I had in mind.
As for the actual pregnancy that I want to happen, not sure what you mean by viable, but I'll keep an eye on that Sim.
Thanks!

Yes, the spawning of the child in an attempt to resolve the pregnancy would have happened instantly upon re-entry into the homeworld, not that anything happened while you were away. By "viable" I meant a pregnancy that was really progressing and not stuck or something. And you're welcome, as always.

Ok, I didn't let the clock run long enough yet to see if it still progressing as it should but with the pregnancy controller, I should know sooner than later.
Perfect then. Thanks again!

I've been having this happen in my current game repeatedly. The active household has 2 parent sims and a bunch of children and teens. There had already been 4 or 5 children who had already grown up to YA, gotten married, and moved out. At least 2 of those inactive households had a pregnant sim at the time I enrolled some of the active household to University. When the the active household returned home there were extra child sims in the inactive households which had had a pregnant sim before travel. The pregnant sims were still pregnant and later gave birth. I wasn't fully sure what was going on at the time so I left the children alone. I think this happened at least one more time giving me 2 more extra children in the homeworld for a total of 4 unexpected child sims on the family tree.

Honestly, the only reason I picked up on this was because the 2 parents in the active household had saved grandparent related wishes and I was checking the number of grandkids in the family tree. I was also using MC to get some of their married YA (now inactive) children pregnant to satisfy the grandparent wishes.

Later I got the 2 active parents pregnant again and sent one of the teen sons to Oasis Landing for a bit. When the son returned the 2 parents (gay male couple) were still pregnant but there were 2 new child sims at the front door. They had facial sliders all set to 0, skin colored texture over their clothing, and both were bald. I seem to recall that just the everyday outfit was corrupt, not all of them. The extra children added to the inactive households had appeared to be clones of each other, but their outfits did not appear to be corrupt.

With the two extra child sim added to the active family I finally put it together: "travel + pregnant sims left behind in the homeworld = extra children on return to homeworld in those households." I TA'd all the extra children I knew about (6 total, as I recall) in the homeworld. The extra child sims did not seem to affect the pregnancies.

At the time this was going on, I was using a testing version of Traveler. I rolled back to the release version after TA'ing the extra child sims. I have no idea if Traveler was the cause or not, I just didn't know what else to do. I current have more of the household at university but I don't recall if anyone in the homeworld is pregnant. So, I don't know if I still have the issue. FWIW, I was using a CC world for the homeworld for the first time.

Thank you for your input, TreyNutz.
I also use the testing version of Traveler, and I also play a CC world. I recall now that I did see at some point a few kids in town with that awkward skin colored broken outfit, and I was wondering when they arrived because my game is set to ask me if I want to rename the newborns, and they had names I didn't remember using.
Luckily, Eliane's the only pregnant Sim in town right now, so I can keep track easily.
Since my adventurer Sim is on a rampage, I made her travel to Egypt (in and out for the opportunity) right after she came back from France and I had TA'd the kid.
When she came back, there was now 2 kids. The one I TA'd before leaving, and a new one. And she still pregnant. At this rate, my town will explode sooner than later.
So I TA'd both of them again, saved and exit so I could use the stable version of Traveler before returning to France to finish this opportunity and see if the testing version of Traveler is indeed involved in this.
I guess I'll see when she returns if there's now 3 new kids in my house.

It does look like I may have caused this. The game thinks the homeworld is a vacation world to simulate aging and pregancies in vacation worlds get magically progressed because they have no way to progress otherwise. Will resolve by gating the magic to EA vacation worlds again.

Still being pregnant is actually an error though. It must have busted out somewhere.

I'll be interested to hear what happens. I haven't played in a while and am really not sure if I'm going to continue this particular save (for reasons that have nothing to do the mysteriously appearing children) or start a new game for a challenge I'm contemplating.

Oh, that comment was for Mellye!