What is the global maternity rate?

Could you please elaborate on the global maternity rate? Is this referring to the number of women who give birth worldwide annually? Or perhaps it's the average number of children a woman has in her lifetime? Additionally, how is this rate calculated, and are there any significant factors that influence its variations across different regions and cultures? I'm particularly interested in understanding the current trends and any potential challenges that may impact the global maternity rate in the coming years.