Baby measuring 3 1/2 weeks bigger

Emily
I'm 31 weeks pregnant but my doctor said he's measuring 34 1/2 weeks. He's my first so I'm not entirely sure what this means. The doctor didn't act like it was a bad thing. My baby shower is September 10th and he's due October 19th. Should I be worried he may be attending his own baby shower? 😳 (in other words be born early) IVE ALSO ONLY GAINED 8.5 POUNDS