After age 5 years, when compared to typically developing boys, boys with Klinefelter Syndrome may be slightly:
- Taller
- Fatter around the belly
- Clumsier
- Slower in developing motor skills, coordination, speed, and muscle strength
- Smaller testes and penis
- Breast growth (about one-third of teens with KS have breast growth)
- Less facial and body hair
- Reduced muscle tone
- Narrower shoulders and wider hips
- Weaker bones, greater risk for bone fractures
- Decreased sexual interest
- Lower energy
- Reduced sperm production
- Infertility: Nearly all men with KS are unable to father a biologically-related child without help from a fertility specialist.
- Small testes, with the possibility of testes shrinking slightly after the teen years
- Lower testosterone levels, which lead to less muscle, hair, and sexual interest and function
- Breasts or breast growth