pregnant woman; expectant mother
A neutral noun referring to a woman who is pregnant; often used in medical, public-service, and everyday contexts.
I gave up my seat on the train to a pregnant woman.
This medicine cannot be used by pregnant women.
A young man gave up his seat for a pregnant woman on the train.
Pregnant women should avoid alcohol and tobacco.
This hospital has facilities that are considerate of pregnant women.
妊娠 means 'pregnancy' or 'to become pregnant' (used with する) and refers to the state, whereas 妊婦 is the noun for the pregnant woman herself.
産婦 refers to a woman who is giving birth or has just given birth, while 妊婦 refers to a woman who is still carrying the pregnancy.
A Sino-Japanese compound: 妊 is associated with pregnancy, and 婦 means woman or wife. This is a component explanation rather than a detailed historical derivation.