The chances of getting pregnant in any particular month among couples who are trying to conceive ranges from 15 to 25 percent, according to WebMD; thus, couples having unprotected sex run the same risk of pregnancy. WebMD notes that the are several factors that can reduce the chance of pregnancy, however, including age. Once a woman reaches age 30, her chances of conceiving a child begin to diminish and, by her 40s, there's a steep decline in her chances of conception.
Women with irregular menstrual cycles also have a lessened chance of conception when actively trying to become pregnant. This is due to the difficulty of predicting ovulation in a woman with irregular menses.
WebMD notes that women who have been trying to conceive for a year or longer without a pregnancy occurring may have a lessened chance of getting pregnant. After one year, WebMD recommends talking to a medical professional about testing for both male and female fertility problems.
WebMD also states that women should pay special attention to their particular menstrual cycles in order to recognize symptoms of fertility and heighten the chances of predicting ovulation. Because the length of cycles can vary from 21 days to 35 days or even longer, women who understand their cycles start and end dates can be more successful in figuring out the best time for conception.