Is a pregnant woman able to "suck in" her stomach?

Yes and no. : Depends how pregnant she is.

Pregnancy changes: Once the uterus is too large, you won't be able to "suck in" like you could before pregnancy. The rectus muscles (your six-pack muscles) stretch and separate during pregnancy, especially later in pregnancy. These are the muscles that women "suck in" and one can't do this as easily in later pregnancy.