妊娠

妊娠している　= pregnant (adjective)

Examples:

• My wife is pregnant with our second child.
• Kumiko is 8 months pregnant so she's started maternity leave.
• I can't drink alcohol because I'm pregnant.

Expecting = pregnant (+ birth date) (not duration)

Examples:

• My wife is expecting our second child.
• I'm expecting so I can't drink alcohol.
• Mayumi is expecting twins in April!
• Kumiko is 8 months expecting. X 

Idiom:

Hayato's girlfriend's got a bun in the oven.

= Hayato's girlfriend is pregnant.