Question

Why does a mature human female generally bear one child at a time?

Answer 1

• Mostly, there is only one egg released by one ovary per ovarian cycle. Occasionally when two or more eggs are released then fertilization does occur as only one egg survives to become a child.
  
• Eggs also prevent Polyspermy by a change in membrane potential after imbibing one sperm in an egg(fast reaction) or by a change in the molecular composition of Zona pellucida after the sperm enters(slow reaction).
• So, this is why most human females bear only one child at a time.