A woman's fertility declines after the age of 35. The probability of achieving a pregnancy naturally is only about 5-6% after the age of 40, and at the same time, the risk of miscarriage increases.
A woman's age is one of many risk factors for pregnancy. Other risk factors for pregnancy are:
Chronic diseases such as diabetes or arthritis
Unhealthy lifestyle (poor diet, too little exercise)
Use of medicines
Alcohol, nicotine and other intoxicants