Fans on both sides rose to their feet to applaud the Portuguese forward at the 7th minute - the same number on his shirt.
Ronaldo scored his 100th Premier League goal in the 33rd minute of the game at Arsenal's Emirates Stadium and paid a poignant tribute to his unborn son by pointing to the sky.
The milestone goals mean he is now the 33rd player to reach the landmark, and the fourth this season alone.
The 37-year-old has become the first player in history to score 100 goals in both the Premier League and La Liga - Spain's football league.
He and his partner, Georgina Rodríguez, announced they were expecting twins in October 2021.
But in a statement shared on social media on Monday, the couple said they are "devastated" at the loss of their baby boy and added that Ms Rodríguez gave birth to a girl.