Differentiating: 肚 and 胃 in English?
肚 is belly, 胃 is stomach. So:

啤酒肚: Beer belly.
肚子疼: Pain in the belly, could be the stomach, but may well be the appendage.
大肚婆: Slang for pregnant woman.
小肚子: The underbelly. Formal: 下腹 or 小腹.
肚脐: Belly button.
肚量大: Big-hearted. (Funny, but we also say 心胸宽广.)

When you read it as dǔ, it means the stomach of an animal: 猪肚, 牛肚, etc. Usually edible by Chinese standards. Sometimes you hear 肠子肚子 which means &quot;stomach and intestines&quot;. A human stomach is NOT called 人肚.

Update，肚子疼 vs 胃疼：

病人：医生，我肚子疼。
医生：你肚子哪里疼？
病人：这里。（Points at the left）
医生：那应该是胃疼咯。

So 肚子疼 is the pain in the belly area, maybe it's the stomach, maybe not. 胃疼 is more specific, it is the stomach, no doubt about that. Also:

女雇员：老板，我肚子疼，今天请个假。
男老板：那你去医院看看吧。
女雇员：不用的，休息休息就好。

In this case 肚子疼 means &quot;period pain&quot;, and the boss should say things like &quot;好好喝水，多多休息&quot; instead of advising her to go to the hospital.