The steroids around when you're pregnant make your bones grow a little bit. In addition the relaxin hormone which allows your pelvis to open for the birth also relaxes all your other joints, including the ones in your feet, which, combined with the extra weight of the pregnancy, allows them to "spread" a bit too. There is also residual swelling which will go eventually. I was a UK7 before PG and am now more like an 8 (SOME 7's fit, but not many). It rarely changes much again after the first baby and there isn't really a way to shrink them back. If i were you i'd give it 4-6 months to make sure all the pregnancy hormones are gone and then see if they fit. If they don't you need to get bigger ones. At least it's summer coming, so thongs are ok

Bx