Depends on many factors - the woman’s physique, posture and weight, the size of the fetus, and the condition of the abdominal wall. Normally, a small bulge below the navel is noticeable by 11-12 weeks, but variations are possible. If a short, fragile girl has multiple births or a large fetus from a large father, the belly will become visible a little earlier; if the expectant mother is full or her belly itself is convex and round, a little later. But by 23-24 weeks it is already difficult to confuse.