I was really interested as to what one of these was so I went to a website that sells Mexican bolas and this what they had to say:

What is a Mexican Bola

The Mexican Bola is a chime ball that was first worn by pregnant women in Mexico who learned through generations of motherhood that the soothing sound of the Mexican Bola against the tummy had a calming effect on their babies before and after birth.

Mexican Bola Pregnancy PendantThe beautiful relaxing sound from the Mexican Bola is produced by tiny chimes inside the pendant.

The Mexican Bola is now a worldwide phenomenon as more and more mothers are experiencing the benefits that the soothing chimes from the Mexican Bola has on their babies.

How does it work?

At about 20 weeks pregnant, a baby can hear external sounds through the mother’s tummy. The mexican Bola is worn on a long necklace and placed low on the tummy where the baby is situated. The melody that the Mexican Bola produces while the mother goes about her daily life will gently relax the baby.

Pregnancy BolaWhen the baby is born, the same sound of the Mexican Bola is said to bring them comfort and familiarity. It also makes a great nursing necklace as the baby will love to fiddle with the Mexican Bola while nursing. It also keeps the baby alert and assists effective nursing.

Much has been said about the benefits of playing music for your baby while pregnant but most people stop playing the music once baby is born. This is counter active as the main reason to play music while pregnant is so that once baby is born into the big wide world the same music/melody will be familiar and as a result calming. Hence the benefits of wearing a Mexican Bola.

(Source: Mexican Bola South Africa)