Does anybody know

Does anybody know why my belly doesn't pop untill I'm like 7 months pregnant my last one didn't look like she was there until I hit 7 months but she was heathy weight my whole pregnancy and came out heathy it's happening again with this one, I'm 18 weeks an I don't look like I've gained a pound but the baby is heathy and at a normal growing weight, I'm 120 pounds before I was pregnant I was 116