Luke writes the books of Luke and Acts to a man named Theophilus. Apparently there are lots of stories about Jesus circulating, and he decides to track down the people who actually witnessed the events so he can be sure to record the truth.
The chapter records two miracle birth announcements and two people praising God for those births. John the Baptist is born first. He is born to a couple who had not been able to have children. Jesus’ conception is even more of a miracle since he is conceived by the Holy Spirit.
When these events occur, people were not used to miracles happening. Zechariah didn’t believe the angel, and it cost him his ability to speak. Mary’s story would not have been believed by many. Would you have believed the angel or the story of a teenage girl who is pregnant and says she is a virgin? Believing God is at work isn’t always easy.