Shhh, listen closely. If the sound of your biological clock ticking down to zero is so loud it's practically defeaning, you're not alone. Many women are postponing childbirth and marriage in order to have successful careers, but if you're in your 20s or 30s and childless, you might be worried about your ability to conceive later in your life. Here are the facts. It's no secret that your 20s is your most fertile time. It's usually easy for women to conceive between the ages of 20 and 29, and the rates of miscarriage are low - around 10 percent - for this age group. While starting a family is rewarding, some young mothers miss out on the carefree years of young adulthood.

However, if you're only 50 when your kids are off in college, you'll have plenty of time to enjoy yourself then. In your 30s, it's generally still easy to conceive - especially before age 35. This is a popular childbearing age for women who have established careers, enjoyed their youth and feel ready to raise a family. After 35, however, fertility begins to decline - and takes a nosedive at age 38. As you approach your 40s, your risk of having children with birth defects or chromosomal abnormalities also increases, although many women age 40 beyond have successfully had healthy children. While being older when you start a family gives you more wisdom and maturity, it also means you'll be nearing 60 when your kids finally leave the nest. Of course, there's no right or wrong way to start a family. So silence that biological clock, because you should only start thinking about kids when you're happy, stable and financially ready to handle the burdens of raising a child.