婴儿 (yīng'ér) is the neutral, clinical word for a baby or infant, typically a newborn up to about one year old. 娃娃 (wáwa) is more affectionate and informal, often used for a toddler or as a term for 'doll'; 宝贝 (bǎobèi) is a term of endearment ('darling') rather than a literal infant.