Factors
Age
It is a well-known fact that fertility is highly dependant on the age of the woman. After the age of 35, fertility starts to decline. After the age of 40 this decline is dramatic and it is very difficult to get pregnant after this age, even with fertility treatments. Most of the reports of celebrities getting pregnant at age 45 or even 50 result from the use of other women’s donated eggs.
Sperm
Insufficient numbers of sperm or sperm that have abnormal shapes or slow movement may be the cause of infertility, which is why it's important for any male partner to be included in the evaluation of infertility. The doctor will ask about cigarette smoking, trauma to the testicles and other things like past chemotherapy or testicular radiation that may affect the sperm count. He or she will then ask your partner to do a simple test called a semen analysis to determine the number of healthy swimming sperm.
Ovaries
A woman's ovaries make the eggs that join with the sperm to create a baby. Every month you ovulate, which is when your ovary releases an egg. The egg and sperm usually meet in the fallopian tube where fertilization occurs. Pregnancy results if the embryo moves into the uterus and implants. When this happens you usually miss your menstrual period. Some people don't ovulate regularly or don't ovulate at all for many different reasons. The best way to know if you are ovulating is by determining if you have regular periods. "Regular" doesn't mean having a period every 28 days exactly. It just means that you have a period about once a month (give or take a couple of days). If this is the case then you are probably ovulating. If you're skipping periods or bleeding every day for a month then you are probably not ovulating regularly. If you think you may not be ovulating regularly then you should talk to your doctor in order to determine the cause and possible treatment options.
Sexual intercourse
Sexual intercourse is obviously an important factor in achieving pregnancy! When and how frequently you have sex can help increase your chances of conceiving. The best time to have sex if you want to get pregnant is the two days before you ovulate and the day you ovulate. If you have regular cycles then you can calculate the best days for intercourse. Count the number of days from the first day of menstrual bleeding one month until the first day of menstrual bleeding the next month. By subtracting 15 days from this number you can determine the best day for intercourse in future cycles. If your menstrual cycles are slightly irregular you can do the same calculation for the longest and shortest cycle and determine a “window” of several days when intercourse is most likely to work. You can also find out when you are ovulating by buying ovulation tests at your local drug store. You can also learn other methods like measuring body temperature and checking cervical mucus. You don't have to have sex every day, 3 times a week is usually enough. Many lubricants (including saliva) will kill sperm, so if you want to use a lubricant be sure to purchase one that is not spermicidal.
Fallopian Tubes
The fallopian tubes carry the egg from the ovary to the uterus, where the baby grows. If your tubes are blocked then an egg can't meet a sperm. We don't always know why tubes are blocked, but sometimes it is because of a past infection like Chlamydia or Gonorrhea and sometimes it is because of surgery. Your doctor might order a special test called a hysterosalpingogram (HSG) or a saline infusion sonogram (ultrasound) to determine if the tubes are open.