It's quite common for first mothers. I was 4 months when I knew I was pregnant and even the doctor has to make me take pregkit to be sure (I have ireg mens so I know late).
Until 6 months I don't look like pregnant and can still wear my skinny jeans but it starts to show around late 7 months.
I know it doesn't applies in other omegaverse but I read male omegas tummies are way more thinner than girls when they are pregnant compared to girls.
Tbf, there are women who didn't even know they are pregnant until they get into labor (something to do with inverted uterus and longer torso)
one model i know was pregnant for months and her stomach was flat as my boobs. it's possible
the baby has arms and legs but his stomach is flatter than paper