While their first babies were often conceived with little effort, a full 15 percent of couples trying for their second child have fertility problems. "It comes as a shock," says Sandra Ann Carson, M.D., a reproductive endocrinologist in Houston and president of the American Society for Reproductive Medicine. "Secondary infertility is as much of a problem as primary infertility."

• Obstacles to conception The causes of secondary infertility are similar to those for primary infertility. Oftentimes, there may be a problem, such as irregular ovulation, that was only a borderline issue before the first pregnancy, but has since worsened. New conditions, too, can crop up between pregnancies, such as declining ovarian function, blocked fallopian tubes, low sperm count or motility, and the most significant factor: age.

• Improving your pregnancy odds Before sounding any alarms, there are several steps you can take on your own to up your chance of conception. First, if you're still breastfeeding your firstborn, you might want to wean  -- at least partly. If a nursing mom's menstrual cycle hasn't resumed, she's unlikely to get pregnant.

It seems obvious, of course, but the most important thing to do is to have sex  -- lots of it. Women are more likely to conceive when having sex on the days leading up to ovulation, and are most fertile the day before ovulation. Timing intercourse with the help of an ovulation kit or a fertility monitor (which detect hormone surges that occur prior to ovulation) can help. Ovulation kits indicate a 24-hour window around the egg release, while pricier fertility monitors can highlight the weeklong fertile period that occurs before ovulation.

To improve your overall reproductive health, you and your partner should stay away from tobacco, recreational drugs, and natural herbs (which can disrupt your cycle). It's always best to reduce your alcohol intake. And when it comes to exercise, moderation is key. Excessive exercise can lead to amenorrhea (a cessation of periods), so limit workouts to one hour every other day. Being too heavy or too thin can also affect your fertility, so talk to your doctor about getting to your healthiest weight for pregnancy.

• Getting help For those under 35, doctors advise trying to get pregnant for a year before seeking help. If you're 35 or older, don't wait longer than six months. Most initial fertility tests can be performed by your ob/gyn, and she can arrange a semen analysis for your partner. If no diagnosis is apparent, she'll refer you to a fertility specialist. For more information, contact the American Society for Reproductive Medicine (www.asrm.org) or RESOLVE: The National Infertility Association ( www.resolve.org; 888-623-0744).

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