Is it okay if my baby bump doesn't show up until week 20?

You're lucky then. It is not important to show. It is important whether baby is growing well. People who have first child usually don't show the bump as much because the muscles are still firm. For second and subsequent pregnancies, you may show early because the skin is loose. If a person is fat, you will less likely show your baby bum. Skinny people show their bumps more, especially if you wear tight clothes. Most people cannot hide by 16 weeks with some tummy fullness. By 20 weeks, most people can see it.