How early does your stomach get hard when pregnant?

From personal experience I can tell you that it varies from person to person. It depends somewhat on how much fat you carry on your belly too. Very thin people may notice the hardening sooner than someone as chubby as I am. But bear in mind, the fetus is very very tiny and you probably can't even feel your uterus when you lay on your stomach until your 2 or 3 months pregnant at least. So like I said though, if you are particularly thin, you may be able to feel a tiny knot much sooner than that, which is your uterus enlarging and firming up. Your whole stomach doesn't get hard until the baby is much much bigger, much further along in pregnancy and its running out of space in the uterus.