You should expect everything a mum of any age should expect. Lots of those things are lovely but a lot of them suck. If you want to be a parent at a young age, then that is actually, in terms of nature and biology, how it was intended. I don't see it as an illegitimate choice. But you need to be prepared for the realities of motherhood, and once the baby is out, Mother Nature is no longer on our side, unfortunately...
Your body is going to look very different. Bodies don't usually just bounce back into shape. You might have tiger stripes that stay for years (mine go down across my pubic area). You might have new moles appear. You will need to say goodbye to skinny jeans or skinny anything for a while.
You can say goodbye to late nights out and lay ins. Expect to be woken regularly in the night. Even once they are sleeping through the night, expect to be woken at 6 or 7 am on weekends to make breakfast and watch peppa pig.
Your home will be full of children's stuff. Usually lying around everywhere. If you like a tidy home, prepare to tidy up 5 times a day.
Your home might get infested with silverfish or ants, because kids leave little crumbs everywhere. Or you need to spend a lot of time and energy instilling a super strict mealtime routine.
Rainy Saturdays are still spent outside on a cold wet park bench while your kid burns some energy at the playground. Or you spend it in a loud sweaty soft play paying extortionate rates for crappy coffee. The only other alternative is a trashed house and hours of tidying.
Prepare for friends with no children to slowly drift away from you. They will be replaced by other mum friends. They will hopefully be lovely, but maybe not your usual first choice. They just go to the same playground as you do.
Your relationship will fall under immense strain. Arguments arise due to lack of freedom/sleep. If you aren't on the same page about every aspect of parenting (almost impossible) you will have struggles about which way things like discipline, routine, money, affection, etc. should be done.
You will have this lovely new human. Initially they smell AMAZING and want cuddles all the time. When they reach toddler age they just smell of sick and poo. You will spend less time cuddling and more time chasing them around so you can get them dressed.
You will get a lot of judgment. Likely more so because you are a young mum, but not the only reason. Becoming a mum just opens a door to criticism and know-it-alls. Whatever you decision you make, someone will stroll along and tell you that you are doing it wrong.
Everything: money, holidays, furniture, sex life, food, routines, social life, entertainment etc will revolve around the child. You can say right now it won't be like that. But eventually it just happens because you realize anything you do that isn't in the best interest of your child backfires.
That's just a bit of reality as I've been woken up at 6am on Saturday morning, emptied the silverfish traps, chopped up a watermelon in small toddler sized triangles, peppa pig on in the background, saggy skin hanging over my pj bottoms, and wondering which playground we can go to once I've spent hours negotiating with my dd to get dressed.
I love her to bits, but at 16 I was happier doing so many other things.