Birthday has many variants in ASL. For purposes of Baby Sign Language, we choose one of the easiest and most common variants. You take your dominant hand, with fingers open and middle finger extended forward. Touch your middle finger first to your chin and then to your chest.
First and second birthdays are a big deal for parents, but babies don't always understand the significance. You can help babies prepare for the event by going through picture books about birthdays and learn about games, and cake, and candles, so they are ready when the big day comes.
This is an advanced sign best used once your toddler begins gaining an appreciation for birthday parties.