Each woman's oocytes (eggs) supply is finite, which means the body doesn't produce new ones. Each female is born with around 7 million eggs, and by the time she hits puberty, only about 250,000-300,000 remain in her ovaries. With each menstrual cycle one egg is released, and an additional 1,000 eggs each month are lost through a process called artresia, the natural breakdown of the eggs by the body. After ovulating an average of 400 times through her life, typically at around 50 years of age, the store of eggs is tapped out. That's menopause.
Eggs age along with the rest of the body. The older eggs are more likely to have chromosomal abnormalities, making them unlikely to become viable embryos. It's important to note that a fertilized egg with abnormal chromosomes is the single most common cause of miscarriage. As a general rule, women in their 20s have about a 20% chance of having a miscarriage each time she becomes pregnant, a woman in her 30's, a 30% chance, and a woman in her 40s, about a 40% risk of miscarriage.
The bottom line is the older we get, the less likely we are to conceive and have a successful pregnancy. Fertility starts to decline when a woman is in her 20's but when she hits 35, it takes a sharp downturn. At 40, fertility falls off even more dramatically.
Of course, some women in their late 30s and a few in their 40s conceive effortlessly, carry and deliver healthy babies. But the likelihood of that happening without medical intervention becomes more remote with each passing year. For women under 30, the estimated chance of becoming pregnant in any one cycle is between 20% and 30%. When women turn 40, that probability plummets to approximately 5%. Even more significant is that when a woman experiences difficulty conceiving in her 40's it is a far greater challenge to achieve a live birth using her own eggs even with the best medical technologies.