Infertility generally means an inability to get pregnant after one year of trying (or six months if a woman is over 35).
The inability stay pregnant is an additional situation that falls into the category of infertility too.
It's actually quite complicated to get pregnant which may seem odd considering its commonness around you:

• An egg must be ejected from a woman's ovaries (otherwise known as "ovulation").
  
• The egg is required to pass through the fallopian tube to the uterus (womb).
  
• Sperm must then fertilize the egg along the way (join up with the egg in other words).
  
• The fertilized egg has to then attach itself to the inside of a woman's uterus (otherwise known as "implantation").
  

Problems in one or more of the above areas may mean a person is infertile.
Fertility is fairly common among women and it's estimated that around 10 percent of women in the United States between the ages of 15-44 have some degree of difficulty getting or staying pregnant (according to the CDC for 2011).
Figures for other developed nations are fairly commensurate.
One important point to remember is that infertility isn't just a woman's problem.
Women and men can have problems with it.
Roughly one-third of all infertility cases are related to the woman's body; one third are due to male infertility problems; and the remaining one-third are caused by varying mixtures of male and female problems or by unknown factors.
That last case doesn't offer much hope.
For women, most cases of infertility are caused by ovulation problems.
No eggs will be fertilized if this process isn't functioning properly for her.
Irregular or non-existent menstrual cycles are good indicators that a woman isn't ovulating properly.
Polycystic ovarian syndrome is sometimes the cause of ovulation problems.
It is caused by a hormone imbalance that often interferes with the normal ovulation process.
This is the most common cause of female infertility, but there are others.
Primary ovarian insufficiency is another one which occurs when a woman's ovaries stop working as they should before the age of 40.
Most common things that affect a woman's ability to get pregnant are:

• Her age
• Whether or not she smokes
• Level of use of alcohol
• Level of daily stress
• Poor or inadequate nutrition
• Not exercising (or exercising too much)
• Being grossly overweight or underweight
• Possessing sexually transmitted diseases or infections
• Health problems that cause hormonal changes (such as the syndromes mentioned above).
  

It is often a good idea for women to talk to their doctors before trying to get pregnant.
Doctors can help you get your body prepared for the task of bearing a child, and they can offer advice on the conception process.
To find out if a woman or man is infertile doctors perform an infertility checkup.
A physical exam is usually required followed by a complete log of the partners' health and sexual histories.
In some cases this locates the problem, but in other cases more tests are needed.
In women, the first step is to find out if she is ovulating each month.
One of the easiest ways is to use a home ovulation kit which can be bought at drug stores.
Blood tests are another method of checking for proper ovulation.
Finding the cause of infertility can be a long and emotional process and may take some time.
Treatments for infertility are various, expensive and specific to different situations: medicine, surgery, artificial insemination and assisted reproductive methods can all be applied to individual needs of couples.
Combinations of these treatments are also possible and to varying degrees, successful.
Infertility is often treated successfully with surgery and/or drugs.
Physical problems in women can sometimes be corrected with surgery but fertility medicines are used to treat women with ovulation problems.
Your doctor should advise you of the dangers and benefits of taking these medications as some can be harmful.
Do some research and understand the side-effects before taking any infertility medicine.
Alternative medicines are another growing trend away from traditional (and perhaps harmful) medical treatments with some couples claiming benefits and success with some of them.
Traditional Chinese medicine offers what it claims to be centuries old "cures" for infertility.
Check these out thoroughly before taking on such a program if you're interested in more natural, holistic and (perhaps) less harmful approaches.
Interview people who've done it for their recommendations.
One last thing to remember is that traditional medical fertility drugs increase the chance of having twins, triplets or even more offspring.
Multiple fetuses can cause a women to have more problems during pregnancy, and there's a much higher risk of premature birth.
Babies born prematurely generally have a higher risk of health and developmental problems too.
Know all the side effects before trying.
Stay healthy, consult your doctor before trying to conceive, do your own research about the options available to you and you'll come that much closer to becoming a successful mother or father.