I'm 17 weeks or so I just started getting a bunch of what feels like little muscle spasms just under my rib cage on the left.  
I can even feel them if i put my fingers there.  I'm assuming it's just a muscle spasm as the location seems a bit odd (shouldn't it be lower?) and my doctor said i might not feel them as soon as normal as my placenta is anterior.
What did the first movements feel like to you?                
                             
        
Re: How do I know if it's the baby moving? or something else?
My OB actually said 16-17 weeks is a little early to feel any real movement but I swear I felt some that early. Now, I know for sure it's the baby. It's usually lower and not around my rib cage at all but babies change positions a lot so you never know.
I'm 18+2 and last night while relaxing on the couch I swear I thought I felt the baby. To be more honest, I felt something "weird" low in my pelvis, and since I'm familiar w/ what gas and indigestion feel like, I'm assuming it was baby. It was a strange, kind of undulating/rolling sensation, followed by a few pokes. Weiredddd (I'm a ftm, so it was a very new sensation). I've felt it a couple more times throughout the day today as well. Still trying to truly decipher if it's baby or not, but the more I feel it, the more I think it is!
I agree the location (up toward your ribs) seems like an odd place to feel baby. According to what I've read the uterus hasn't even reached belly button height yet, so maybe it's something else?
DS2 due 12/12/18
Good luck, it'll happen any day now!!
i also have an anterior placenta, but i actually felt my first littl nudge at 17 weeks (a little below my belly button). i didn't really feel him again until 19-20 weeks. you'll know when you feel baby move. it's a feeling like no other!
I'm almost 26 months now and don't confuse the baby moving at all...it's very much obvious at this point. I think when you know, you really know.
Me: 42, DH: 46, Married: 11/12
Losses: MMC#1 11/12 BO, MC#2 11/13 at 8w BO?, MMC#3 8/14 chromo healthy M @12 weeks, stopped growing at 10.
Negligible AMH, FSH finally went high. Pursued DE.
DD born at 38w2d on 5-27-16. Finally!!
Pregnant again with OE. EDD 11/9/17 Girl!
Me: 29 / Hubster: 31
Married July 2010
DC #1 Oct 2013
DC #2 EDD June 2016