Time Enough For Love, “The Tale of the Twins That Weren’t” IIRC.
And my daughter knew from the git go that I was pregnant with her when her dad and I married–since I dropped out of high school and we married in April and she was born in July it wasn’t exactly a Big Secret. She’s fine with it, and never did marry the father of her son, thank goodness. Guy’s an asshole.
It didn’t work for us. Our first pregnancy took nearly two years.
Our SIL was born 4 months after his parents married. I don’t know when he learned this, but it never bothered him and he has made no attempt to conceal it. He was born in 1969.
Speaking from personal family experience, how things get told probably depends upon how the parents feel about the circumstances. I have a half-brother who is 14 years younger than I am. My mother and her S-O (my father and mother had been divorced about 7 years by then) had been together for about 4-5 years at that point, but had not really intended to start a family. This would have been in the early 90s, so abortion was an option, but if they discussed it, they didn’t with me then or now.
Whatever options that were on the table, they did decide to get married with minimal fuss, but everyone in the family knew the circumstances. The S-Os family was . . . much more traditional, but nothing was said, because they were a traditional rural family that had already had 2 girls in the family in High School get married for similar reasons.
Sadly, my step-father did not take being married well. While I do think he loved my mother, he felt that he had been collared, probably aided by the fact that in the 80s, he’d been a big resort real-estate guy with a lot of easy cash, and by the 90s the market was mushy, and he was doing other work and no longer could play - plus the expenses of a child (I will say he always treated me well when we would visit over the summer, so that’s something).
So while he, to the best of my knowledge, never took anything out on my brother, he frequency self medicated with booze, and was at times verbally abusive to my mother for trapping him and similar language - my brother ended up knowing at a rather early age, and I feel dealt with it well.
Of course, we’re all adults now, and he’s successful and happily married. I do wonder if it contributed to his stated desire to not have kids though, which is something I share, but only because my wife and I are selfish with our time and energy. And going back to my original point, if you don’t deal with it well, you may end up like my step-father: he and my mom are separated (too much hassle and money to divorce, but they live in different cities), and my half-brother spends time with her, not with his father.
My niece was born 6 months after my brother got married. When she was a teenager, we had some family party that happened to be around her parents’ anniversary. Someone asked how many years they had been married and my sister-in-law said she didn’t remember the year. My niece mumbled something about how she didn’t want to know. From that I assumed that they had never told her and that she guessed anyway but was uncomfortable with it. It was never brought up again in public.
To be honest, I always thought my sister-in-law got pregnant on purpose. She was 6 years older than my brother, had been married twice and had 3 other children. Her ex-husbands apparently were assholes and my brother was a decent guy, so I think she wanted to make sure she “got” him. Even so, they have been married for 33 years now, though I would not say happily, at least since my niece left home.
OMG! Are you my mother??? 
I’m pretty sure my parents “had” to get married. While I did not come along for more than two years after they were married, at one point I learned my mother had had several miscarriages before and after me. I’m the lucky one who made it through. There are reasons I believe my father had knocked her up, but there is no way to know for certain by now.
You were pregnant for two whole years?
Maybe they’re elephants.
I agree. If you honestly would never have gotten married otherwise, it’s a really different narrative than if the pregnancy was just mis-timed. “My parents didn’t love each other, they just felt obligated” is really different than “My parents were so hot for each other they didn’t take precautions” which is different still from “the abusive hell that is my home life is really my fault, for existing”.
Now that’s a scenario I’d never thought about for anyone.
And an interesting example of a situation prone to some close cousin of survivor guilt.
My daughter was born 6 months after marriage. This was the late 2000s, so I don’t think it’s ever come up or that anyone cares. We live in a somewhat well-off suburb and I think out of wedlock births aren’t that common here, even now. People get married, they just get divorced soon. Not a lot of single parent situations with the kids’ friends but quite a few “dad is off with his other family” or the parents got divorced because mom was impregnated by a bartender or whatever.
Also, somehow the fact that I’ve mostly had a “parenting marriage” seems to make it all a non-issue. No real scandals and I’ll probably get a divorce when the kids are grown.
If the kid made a big deal of it, I’d just say, “Time to grow up, Cupcake, and deal with it.”
Now that I think of it, maybe it’s good that I don’t have kids. LOL
I would guess this. I have several friends who intentionally got pregnant prior to the wedding. One couple didn’t plan to marry, but after “sharing” a child realized they WERE married in fact, and decided to legalize it. Their wedding photos include photos of the baby. Three other couples were engaged, and were old enough to worry about fertility, and decided they may as well start working on making a baby. All three brides were visibly and happily pregnant at the wedding.
That’s not a new thing, either. I’m told that back in colonial times, the financial consequences of infertility were so dire that couples routinely waited until pregnancy to actually tie the knot. Lots of first babies born 6 months after the marriage in the records, so I hear.
Yes, if your parents had a shotgun wedding followed by a miserable marriage, I can see feeling awkward about that. But just the raw fact of “birth less than 8 months after wedding date” doesn’t seem like it’s a big deal to the current generation.
Back in the day, the old people had a saying:
“The first one can come anytime. All the rest take nine months.”
Ha ha. 
This is 2021. Why would anyone tell their kids, and other people, anything but the truth?
My mother’s solution was to lie to us about her wedding date. She also lied about her birthdate to make herself younger. So, when I was about six years old, I calculated that she was 15 when she got married. The next year, I learned that no, she must have been 14 when she got married. The next year after that, I learned that my mother was not a reliable source of information about herself.
I grew up in a very conservative religion, Mormonism, but even then it wasn’t a big deal back even in the 80s when I was a young adult and several of the people I knew were having to get married.
My mother is is as straight laced as they come and she had nothing wrong to say when she told me about my cousin who had to drop out of high school because she was pregnant. Getting married seemed to take away any hint of scandal.
I shared about the great aunt who we t to great lengths to hide it a generation earlier, so obviously some people did care about it at certain times and places but I just do t see how it would have been a problem in most places in America in 2000.
My kids (12 and 10) still love hearing stories about them as babies and small kids so the advice from the OP would still be nice even though it most likely wouldn’t be an issue. My son likes to ask questions such as if adults would be better off not being parents and why did we decide to get married.
I lived in Las Vegas in the 1980s. Which, perhaps surprisingly to outsiders, is the main bastion of Mormonism outside Utah. They were, and are, a very large minority fraction of the total population.
An insensitive joke of the time defined a “Mormon virgin” as a girl who’d only had one abortion in high school.
It appears that pre-marital sex was one of those cats that had gotten out of the bag in a big way, and folks were still coming to terms (heh) with the cultural consequences.
I would have given essentially the advice you did, and I’m a bit surprised that this was much of an issue even in 2000. I knew a number of couples then who lived together, had a kid or two, and didn’t bother with the marriage business till later only as a legal formality. I never thought twice about it; I thought that stigma had died out by the 80s or so.
I do know mores are different in different circles, though.