Women who wait usually have more financial security, more emotional maturity, and more time to commit to their marriages and to their careers before focusing their energy on children. It isn't so much that people aren't interested in having children, but they are just busy with their lives. Many couples have waited until they've established careers, bought a house and fixed it up, and then realized that it's getting late and they're running out of time. Deferred marriage also plays a role, and many more females aren't married until a sizeable chunk of their reproductive lives has gone by. The high divorce rate among Americans also plays a part.
Many couples delay childbearing until their marriages are stable.
Other factors that account for delayed childbearing include economic problems in the baby boom generation, the availability of legal abortion, and effective contraceptives that make postponement easier. The media has generally publicized the virtues of older motherhood and suggested that women can easily conceive in mid-life, maybe even up to menopause. Women who put off trying to have children until their mid to late 30s may worsen their chances of becoming pregnant and lose the chance altogether to become a parent. Over the past 10 or 15 years, millions of couples have had reproductive problems stemming at least in part from the woman's age. Female reproductive organs become less effective with age.