As a woman ages, her number of eggs and their quality drop dramatically, says fertility specialist Mousa Shamonki, M.D., director of in vitro fertilization and assisted reproduction at University of California, Los Angeles' David Geffen School of Medicine.

Conception by the time a woman is in her late 30s and early 40s is difficult, and it becomes nearly impossible after age 45, Dr. Shamonki says.

"Their bodies aren't designed evolutionarily to get pregnant as easily later as when they're in their teens and 20s," she says.

Usually, conception happens when an egg cell is released from a woman's ovary, travels through a fallopian tube, and is fertilized by a man's sperm. The fertilized egg undergoes many cell divisions on its way to the uterus, where it implants and grows.

Embryo quality, however, diminishes as mothers-to-be age, while the possibility of chromosomal abnormalities increases, says Shari Brasner, M.D., an obstetrician and gynecologist affiliated with Mount Sinai Medical Center in New York.

"Biologically, Mother Nature doesn't care what your reasons are for delaying childbearing," says Barry Brock, M.D., assistant clinical professor at UCLA's department of obstetrics and gynecology.

His advice: "If you've found your life partner and want kids, don't wait."

2. Sexually Transmitted Diseases (STDs)
Oh, those college liaisons. So spontaneous, so passionate, so unprotected. Many produce an unwelcome outcome: STDs.

Genital human papillomavirus, or HPV, is the most common sexually transmitted infection. It can result in cervical dysplasia, an abnormal growth of cells and cancer, if undetected, says Dr. Shamonki.

By the end of college, 70% of students have been infected with HPV, adds Dr. Shamonki. That can increase the risk of developing dysplasia. Procedures to remove those abnormal cells sometimes make it difficult to get pregnant or carry a baby to term.