: How can you make an unrealistic setup as realistic as possible? Seems like a broad question, but hear me out. A while ago, I asked how you could verify information, which was mostly reserved

This doesn't seem like a problem with exposition, or that it creates difficult story demands.

It could be that the MC had been living with her friends, been pregnant and give birth, perhaps even weaned the baby. Then something happened, and for her two years smoothly disappeared. Maybe she is on a trip far away with her baby, then suddenly, no baby and two years gone. Not amnesia, just two years blinked away.

By being temporarily away from her friends and normal life, the two-year break will be harder to detect. Body changes during pregnancy should be a handy clue for the MC. Other things, like calendars, could be unreliable. If you push the conspiracy far enough, the place she is "visiting" could have been set up by those splicing her memory.

Someone wanted the two years to be gone. Maybe she was living a two-sided life, and side-B needed to be covered up. Maybe bug-eyed space aliens zapped her with their infernal forgeteratory and took the baby away to start a super-powered hybrid alien-human race.

Only you know, until you write the story and tell me.

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Indirect Research.
It is difficult to investigate compound events; so break them into two events and invent your way out of the rest. One of your events is amnesia, the other is childbirth.
Research the effects of amnesia: It is virtually impossible for anybody to lose two years and think it was a day; they would have to be 100% utterly alone before they disappeared for that to happen. Nobody missed them, they did nothing with anybody, nobody noticed they were gone? I can't even come up with plausible magic for that, it would be too powerful. So if it is true two years seems like a day to her, then her parents, friends, coworkers, siblings, lovers and servants (bartenders, shop keeps, barrista at her coffee shop) can tell her different: "Oh my god! Karen! Where have you been? Do your parents know you're back? Why?! Because we all thought you got kidnapped and probably killed you idiot!"
Second is the effects of childbirth: I think you are glossing over a rather traumatic health event. For pregnancy you can do a lot of research on how that affects a woman's body; particularly breasts and their vagina. For example, 7 Ways Your Vagina Might Change After You Give Birth. e.g. 79% of births cause tearing of the vagina, sometimes severe enough to require surgery.
Nearly all births cause some loosening of the vagina; which women may notice because tampons no longer stay in the vagina as they once did. Some women opt for elective surgery to repair this.
Birth can damage the pelvic floor and produce urinary incontinence (peeing yourself for mild exercise like jumping, laughing, or even walking).
Related to all of the above, some women for years after a birth find orgasms feel weaker, or difficult to achieve at all. Orgasm particularly is powerful rhythmic contractions of muscles in the pelvic floor and vagina that may have been torn and damaged by childbirth; and looseness in their vagina due to childbirth can reduce a woman's pleasure in intercourse or self-stimulation.
By Indirect Research I mean research the components of something you cannot find, and interpolate between them. Also, better imagine the effects of your plot on others besides your MC. Think of how many people you know that might notice if you vanished for two years and then came back. For a typical person in the modern world, that number will be well into the dozens; especially for younger people (that interact with more people than the retired or elderly).

Edit in response to @storbror comment: I did not even consider that she lived through a pregnancy with everybody around her knowing it except for her. A simple calendar on the wall (or Internet) would tell me two years had passed since the last day I remembered. Pile on top of that noticeable changes in my breasts, vagina, tummy, possible stretch marks and in general my own body due to pregnancy. Combine that with changes and newness in other things, like TV shows I like, movies I haven't seen, politics, personnel and decor changes in stores, I know for certain I have lost two years no matter what my memory says.
Thus in my mind this would not make me question my sanity. I would believe I'd been hypnotized or suffered a stroke or amnesia or some other emotional or physical trauma (which I did, it is just a magical version of it).
Thus, the scenario of "I just forgot about a pregnancy and everybody around me forgot" seems so implausible it would definitely break all suspension of disbelief for me, as a reader. The villain would have to be powerful enough to erase or change this specific memory from a hundred minds, not miss anybody, but not powerful enough to erase or change my memory of who I used to be, or counter the physical effects of my pregnancy. That is just too "deus ex machina" to believe.
A girl searching for two lost years is easier to devise: A girl sexually abused by her stepfather runs away at 16, does restaurant work, uses drugs, etc. At 18 she is abducted traveling between cities, say from Seattle to Los Angeles; and wakes up in an alley in LA --- 20 years old, not remembering the abduction or what happened to her in-between. Then she has no friends to notice she went missing, no friends to notice she is back, and although the calender and other global changes will still quickly reveal she has lost two years, it is more plausible she will not immediately notice bodily changes between 18 and 20.

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This is bordering 'what to write', but I'll add some things to @Galastel 's very nice answer, because this could probably help other writers struggeling with similar questions.

1. Characters

Almost any story has more than one character/person that we follow, either directly or indirectly.
You obviously have your MC, but the side characters in your story can be used to explore sides of your MC and the story:

A provoking stranger will teach us how she handles bad manners.
A call from her nervous mother will teach us how she relates to her mother/parents/family.
An old friend will teach us about her youth, if they talk about it, and how she feels about her past.

2. Events

You've got events in your story that your MC doesn't remember - Other people must remember them, unless she's been completely isolated for over 2 years, which seems unlikely, yet is up to you.
You can use side characters to interact with her in a way that makes her realize that which she doesn't know/remember.

If you're aiming for a somewhat creepy story, this'll be a neat trick: If she trusts the person she's talking to, AND that person suddenly starts talking about her recent past, some part of her has to believe that person, even if at first she thinks it's a 'prank'.

3. Physiology / Psychology

I don't know your age, and it doesn't necessarily matter, but:
People age, always (unless you're in some sci-fi/fantasy genre). Some periods of our lives have a stronger impact on how old we physically appear and behave, but especially stress, be it physical or mental will tear on us tremendously.

If your MC has gone through pregnancy (and birth?) she will look different, unless she is magically immune to the physical and mental effects connected to it. Some things you could consider connected to the pregnancy:

As human beings, we're biologically engineered to, at some point, feel mentally and physically ready for offspring (have a child). This is also a part of what most people consider 'maturity'.
This will show itself in various ways; We will look at the world differently - notice babies, perhaps be attracted to different things from what we used to, we may change our appearence to fit what we are suddenly attracted to.
Also, aging in itself alters our appearence. At some 'stages' in our lives, our age can be hard to tell - and a lot of people never 'look their age' - but if your MC has missed 2 years of her mid-twenties, I strongly believe she would notice herself looking different. Especially if she has been through pregnancy and events connected to that alone.

Guidelines/Tips:

I'm 28 (not a tip), and a lot of these days, I feel as if I look 'tired'.
Granted, I probably never really get enough sleep - Who does in this day and age? - BUT; This could simply be my mind's way of coping with the fact that I am actually starting to look older. I am not the young man I became at the end of my teen years.

Have your MC notice small things about her body - Perhaps look up 'aging signs mid to end twenties' to find things she might notice.
Connect that with the people in her life: Does she have siblings or same-aged friends? Does she have a job with colleagues? They must look older too!

If she has lived her life normally, but simply doesn't remeber the last 2 years (I hope you've got a great explanation, even if you keep it to yourself), most of the clues or - to her, discrepancies - will be in her head through the things she considers, notices and realizes.
I don't randomly walk around asking friends or family if they remember 'this and that', if they are obvious things to remember - like being pregnant. However, people like to talk about the fun/odd/lucky/out-of-the-ordinary things that happen that are connected to the obvious things.

A friend might say: "I can't believe your belly i so flat again already. Do you even have stretch marks?"
This, however, would indicate that her pregnancy is something that 'they' can talk about, meaning it is probably not a traumatic event. If she lost her child, you probably wouldn't 'compliment her body of getting over the pregnancy'.

A nice 'trick' you could use is to have her pregnancy be somewhat taboo, meaning that the people that know her, don't talk about it - making it more difficult for her to find out.

So, to sum up:

The 1, 2 and 3 should come together in a web of experiences that makes your MC notice things that are slightly or completely 'off'.

Have your MC meet people that know her.
Have her confused when someone is talking about something in a manner, where it seems she should know what they are talking about - that's usually how people talk to someone they know.
Have her notice small changes about herself and the people she interacts with.

What she finds out is up to you, but using side characters is a powerful tool for showing that something is in a certain way.

EDIT

It has been explained that the MC is not the only one suffering from memory loss. In that case, the MC and the side chraracters could instead notice various changes in eachother first, and then themselves and the rest of the world as a consequence of that.

This could start small, but soon she could find herself in a group of friends, relatives, and perhaps strangers, searching to understand what is happening or had happened for all of them to feel and appear this new way.

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Any "what if" scenario is where your creativity must come to bear. It is for you, as a writer, to consider the questions you ask. The first step is indeed asking the questions. Then you find an answer that seems good to you.

An important element is being internally consistent. The rules of your fantasy are your playground, but they cannot contradict each other.

Now, to answer big questions, I find two things make the task easier:

Break the large question into smaller questions
Create more of the world than goes onto the page.

For your example, while your character does't know what happened during those two years, nor that two years are missing, you should know what happened. Has she made some new acquaintance? Has she bought new clothes that are now in her wardrobe? Got rid of something that she now can't find? Has her clothes size changed? What about her friends? What about technology / fashion - has it moved on? What about newspapers / calendar / anything that lists the date?

And so on. Ask questions, and they will guide you. Some of those questions would lend you information that would end up on the page, others would merely inform your writing, without ever showing up. Knowing more, considering possibilities and implications, is what allows you to make your world both interesting and internally consistent. And if it is internally consistent.

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