Women are most fertile during their 20s and early 30s. As they age, fertility gradually declines, and the chances of conception decrease significantly by the late 30s and early 40s. This decline is primarily due to a reduction in the number and quality of eggs in a woman's ovaries. While it is still possible for women to conceive after this age, the likelihood decreases, and the risk of pregnancy complications, such as chromosomal abnormalities and miscarriages, increases.