For men:

• Age - although men are fertile from puberty onwards until their 70s, their testosterone levels decline at a rate of about 1 per cent each year after the age of 30.
• Sperm quality, quantity and sexual performance also go downhill as men get older.
• Defects in sperm quality or quantity caused by genital tract infection or varicocele.
• Sexual dysfunction such as premature ejaculation

For women and men:

• Smoking
• Heavy drinking
• Over-exercise
• Anorexia
• Obesity

>> Causes of infertility in women

This article was first published in Mind Your Body, The Straits Times.