Pregnancy and Balance
As you progress through into the second Trimester, at some point you will show. Yes you belly will appear bigger. As this happens it will be accentuated by a shifting and counter shifting within your body. Your muscles will adapt by shortening or lengthening to suit.
As your belly goes forwards, your head will follow so your glutes (bottom) must go backwards to counter balance, or you will face plant. As the Glutes become longer, they are also weaker leaving them as a place where pain come start from. Lower back pain in pregnancy is often associated to the glutes being in this position.