"has also become pregnant with a son, even though she is already very old" or "even though she is old, has also become pregnant and will bear a son" Make sure it doesn't sound as though both Mary and Elizabeth were old when they conceived.
"because God can do anything" What God had done for Elizabeth was proof that God was able to do anything, and that he could even enable Mary to become pregnant without her having sexual relations. (See: [[rc://en/ta/man/translate/figs-litotes]])
"I am a servant of the Lord" Choose an expression that shows her humility and obedience to the Lord. She was not boasting about being the Lord's servant.