Captkanga

https://travel.state.gov/content/travel/en/international-travel/while-abroad/marriage-abroad.html
 
https://turismo.buenosaires.gob.ar/en/article/get-married-ba

Both involve having the foresight to recognize undesired outcomes without actually wanting them to happen. Our society finds it so reckless not to plan for such outcomes that it is illegal not to be prepared  in the case of insurance.

The connection here seems rather obvious.   I do realize that everyone likes to think they're better than average, but the fact is that most people are average by definition. So yes, the statistics in both divorce and car accidents apply to you as well. Hopefully that's clear enough to save you from having to wonder anymore. I'm not going to explain it further.

You mean like why buy insurance when you don't plan to drive recklessly? 

A pre-nup is a personal choice and have no bearing on immigration.
 
It makes some sense, as evidenced here with several "my husband/wife left/dumped/divorce me" stories.  There is about an equal mix of USC's divorcing their immigrant spouse and vice-versa.   Marriage is a highly emotional decision.  Even if things are well thought out and the couple have spent a lot of time each other and are committed to each other, there are situations when feelings change or life events causes change.   If one party has significant financial assets, and state laws don't distinguish between pre-marital and post-marital items, then it makes sense to legally make such distinctions.  Sure a pre-nup will cool some emotions, but if it affects the couple to the point where one party feels hurt by it and not recognize the practicality, then perhaps the relationship wasn't built on something that would have lasted in the first place.
 

Too funny.  "bought" me here.
 
A pre-nup covers substantially more than what an affidavit does, including but not limited to pre-marital property. 

There are so many implications when it comes to this.  Besides what people have shared, you have to think about some cultural/governmental things in Argentina.
 
There is a lot of red tape and what would be "easy" here won't be easy over there.  The having a child out of wedlock could get complicated in Argentina and the USA if you are trying to naturalize them by having a US-citizen father.
 
I know getting married could delay it by a month or two, but it might be a smoother ride.  If you are concerned about the responsibility of the I-864, think about the potential of doing a pre-nup and just talk to your S.O. to see if they would agree to become naturalized as soon as they are able, etc.  Again, none of this is a guarantee.  Props to you for being realistic and at least talking about it.  I know some people might jump on the "it is a red flag" if you are thinking about the failure already.  But, I also think that if your relationship is healthy, it is good to just be open and talk about all the possibilities so you could be on the same page no matter the outcome.

We have a Whatsapp group of Argentina filers.  If you are interested in joining, feel free to inbox me.

The father provides ecidence that he, in writing,
 
* acknowledges the child as his
 
* agrees to provide financial support for thr child
 
If the father does so, and successfully pursues a CRBA for the child, great.

Get  married before your child is born.

Child is generally assumed to be legitimate if parents are married. 
if you’re not married you might have to 1.) include proof you were together during the time the child was conceived 
2. undergo a DNA test 
this isn’t a certainty it’s just an extra barrier they might make you complete 
Not continuous. Just 5 years total. 
2 have to be after the age of 14. So say year you graduated high school and 2 years of working on the US after the age of 20. Then maybe 1 year in elementary school… (school records, vaccinations etc) add miscellaneous year you returned from Europe in 2021… 

Good that you asked about this early.  Some may say that thinking about divorce at this stage is unromantic, but life doesn't always go our way, despite our best intentions.  My view is that it's best to enter marriage with eyes wide open, more so a marriage that would involve the I-864.  If you browse through this sub-forum, you will find recent examples of people who realized too late how the I-864 they signed could be used against them.
 

It sounds like you have not filed anything yet, so if you are still planning on an eventual K1 visa, the first affidavit of support will be the I134 at the time of the interview, and to my knowledge the I134 does not ask for household size specifically, but you will need to meet the I864p requirements at that time.  After the K1 is issued a you get married and file the I864 with the I485 AOS submission, you will have to list the total household size which by that time will be at least 3.
 
On a side note, with a child coming, have you considered a spousal visa?  Things like CRBA, etc. can be smoother if the child is not born out of wedlock.  @Mike E can sum it up much better than me, but it is something to think about.
 
Good Luck!

*** Moved from K1 Process & Procedures to Effects of Major Family Changes on Immigration Benefits, where similar topics about I-864 obligation and divorce are discussed ***
 
 
Fill out the I-864 with what is accurate at the time you sign it.  If your child has already been born, then 3.  If not, then 2.  If your dependents increase before your spouse's Adjustment of Status (AOS) interview, bring an updated I-864 to the interview.
 
 
There is no 10-year limit.  You could be bound by the I-864 for your beneficiary's remaining lifetime, if they never work or naturalize, even if you divorce and they re-marry.
 
Note that the I-864 is a contract between the sponsor and the federal government for possible reimbursement of public benefits claimed by the immigrant.  It is not a contract between the sponsor and the immigrant, nor does it dictate how much financial support the immigrant should get, if any, from the sponsor.  Divorce settlements and alimony are matters for divorce lawyers and family court, and separate from the I-864, though some divorce lawyers may use the I-864 as ammunition to negotiate a better settlement for their immigrant clients.