1. Lying about being pregnant has never worked in fiction.* Seriously, I can think of more fictional characters who have gotten away with murder. The reasons are somewhat obvious. There are doctors appointments, sonograms, to say nothing of the physical changes. Sure you could fake these, but only if, say, you plan no actual contact with the folks you are lying to.
2. Pregnancy causes physical changes. Now everyone is different. And sure, I was in math class in high school with someone who was later revealed to be pregnant, but we didn't hang out or interact regularly. Plus people (and I say people, but let's face it, I have yet to see a story about a guy faking a pregnancy...oh, dear, look what you've done to my writer brain now.**) You can try to fake this, but this assumes you are not going to need physical contact with anyone until you're done. Given how many people accost the stomachs of pregnant women without asking, this seems a tricky proposition.
3. Doctor's appointments. HIPAA laws (when fiction remembers they exist) work in your favor in that your medical info is your business, but it's not uncommon for people - say the supposed father - to wish to join appointments, see sonograms, and so on. Sure you can try to talk them out of that, but given the purported reason for such things is often to hang on to this person (which, by the way, whole other can of worms) that seems self-defeating.
4. The baby. Now that DNA and such has made stealing someone else's baby a trickier proposition (plus the whole legality of that) people seem to be working on the we'll just celebrate a lot and then I'll be pregnant and it will just look like a long pregnancy. And look, I certainly can't say that's never ever worked (especially as we are talking fiction) but it seems to defy the law of average pregnancy achievement by a lot. Now sure, there are still plenty of unplanned pregnancies and those typically get counted as first try but, hey, it's fiction, so give it a shot.
5. The not baby. If you have decided that you have gotten as far as you can get with pretending and now need to pretend a miscarriage, well, there are similar challenges. In general, they like you to be examined by a doctor after such things to make sure that all is okay. If you have achieved your goal of closeness with your partner, they would probably like to accompany you. Again, HIPAA works in your favor, but in general I find doctors are not fans of taking up their time with non-miscarriages for fictional pregnancies, so you could argue you don't need care, or can't face it but that seems equally problematic.
6. Yeah, so my recommendation for fictional characters is that fake pregnancies are on par with blackmail, in the fictional folks always get caught, so maybe work on something else.
*Feel free to provide counter examples, my most likely response is going to be, is the story done yet? Are you sure?
**There was that one story about the dad who wiped the pregnancy from his daughter's memory, but that's a little different.
