When you and your spouse are trying to conceive, months of fruitless attempts can feel like an eternity. But a fertility expert may be able to help.

More than six million couples in the U.S. struggle with infertility, which affects about 10 percent of all people of reproductive age. If you're under 35, see a fertility specialist if you've been unsuccessfully trying to conceive for a year; however, if you're over 35, wait only six months, since your chances of conceiving after that age diminish greatly. Your ob-gyn may be able to field some general questions, but it's a good idea to go to a reproductive endocrinologist, who has more expertise with infertility.

There are many possible causes of infertility. About a third of cases can be attributed to male factors, another third to female factors, and a final third to a combination of both partners, or are simply inexplicable. Here, some of the most common conditions that cause infertility:

1. Ovulatory disorders: A condition in which the egg isn't properly released, or isn't released at all, from the ovaries.
   
   Prevalence: Occurs in about 40 percent of all infertility cases.
   
   Diagnosis: Your doctor may be able to tell if you have an ovulatory disorder by analyzing your menstrual history. Irregular periods or a complete lack of menstruation may be the clue, although some women with normal periods can also suffer from ovulatory disorders. Your doctor may also perform a blood test at certain times during your menstrual cycle or an ultrasound to clarify whether ovulation is occurring.
   
   Treatment: Usually consists of fertility drugs that stimulate ovulation, like Clomid and Follistim.
   
   
   
   
2. Blocked fallopian tubes: The fallopian tubes may be either totally or partially blocked, slowing or completely preventing the flow of sperm to the egg.
   
   Prevalence: Occurs in about 30 percent of all infertility problems.
   
   Diagnosis: X-rays can usually reveal if the fallopian tubes are blocked. Your doctor can also use laparoscopy, which involves inserting a narrow telescope-like instrument through a small incision below your navel to view your reproductive organs.
   
   Treatment: Surgery to remove the obstruction or in vitro fertilization (IVF). IVF involves removing eggs from a woman, fertilizing them with sperm in a laboratory, then transferring them into the uterus a few days later.
   
   
   
   
3. Sperm disorders: The sperm can be abnormally shaped, unable to move properly, or too few in number. Infectious diseases, such as mumps, or an inadequate production of certain sex hormones can lead to a low sperm count.
   
   Prevalence: Occurs in about 40 percent of all infertility cases.
   
   Diagnosis: Semen analysis. A semen sample is taken and tested in a lab. If problems are detected, your doctor may suggest a urologist who specializes in male infertility.
   
   Treatment: Most often, artificial insemination, which involves injecting sperm through a narrow catheter into the woman's reproductive tract.
   
   
   
   
4. Endometriosis: A condition in which the lining in the uterus, which usually sheds during menstruation, grows outside the uterus. Endometrial lesions can block the fallopian tubes or impair ovulatory function.
   
   Prevalence: Occurs in about 20 percent of all fertility cases. About 70 percent of women who have endometriosis experience infertility.
   
   Diagnosis: Laparoscopy, a surgical procedure in which a thin scope is placed through the belly button into the abdomen to view the uterus, ovaries, and fallopian tubes directly.
   
   Treatment: Laparoscopic surgery to remove the lesions--laser laparoscopy can remove endometrial tissue and blockages from the body; hormonal medications, such as estrogen and progesterone; or in vitro fertilization.