(HealthDay News) -- Down syndrome is a genetic birth defect that's usually caused by an extra copy of the 21st chromosome.

It's the single most common cause of human birth defects, affecting about one in 660 births, the U.S. National Library of Medicine says.

The NLM lists these common signs of Down syndrome:

• Lack of muscle tone present at birth.
• A separation of joints between the bones of the skull, or an abnormally shaped skull.
• Eyes that slant upward.
• Small mouth with a protruding tongue.
• Wide, short hands.
• Only a single crease in the palm of the hand.
• Abnormal physical development, mental retardation.
• An abnormality in the iris of the eye.