The chance of a perfectly normal couple achieving a successful pregnancy is only about 25% even if they have sex every single day. Fertility depends on the woman's age intercourse frequency trying time and fertility related problems.

When most couples get married they expect to have their own babies. Many naively expect they will get pregnant the very first month they try - and are concerned when a pregnancy does not occur. All of us go through a brief interlude of doubt and concern when we do not achieve pregnancy the very first month we try - and we start wondering about our fertility. 
 

Only a 25% chance!

Before worrying, remember that in a single menstrual cycle, the chance of a perfectly normal couple achieving a successful pregnancy is only about 25%, even if they have sex every single day. This is called their fecundity, which describes their fertility potential. Humans are not very efficient at producing babies! There are many reasons for this, including the fact that some eggs don't fertilize and some of the fertilized eggs don't grow well in the early developmental stage. 

Getting pregnant is a game of odds - it's a bit like playing Russian Roulette and it's impossible to predict when an individual couple will get pregnant! However, over a period of a year, the chance of a successful pregnancy is between 80 and 90%; so that 7 out of 8 couples will be pregnant within a year. These are the normal "fertile" couples - and the rest are "labeled " infertile - the medical text book definition of infertility being the inability to conceive even after trying for a year. Couples who have never had a child, are said to have "primary infertility,"; those who have become pregnant at least once but are unable to conceive again, are said to have "secondary infertility."
 

Factors that determine fertility

The chances of pregnancy for a couple in a given cycle will depend upon many things, and the most important of these are:

• The age of the woman. At the biologic clock ticks on, the number of eggs and their quality starts decreasing.
• Frequency of intercourse. While there is no "normal" frequency for sex, the "optimal" frequency of intercourse if you are trying to get pregnant is about 3 times a week in the fertile period. Simply stated, the more sex the better! Couples, who have intercourse less frequently, have a diminished chance of conceiving. 
• "Trying time" - that is, how long the couple have been trying to get pregnant. This is an important concept. The longer a couple has been trying to conceive without success, the lesser their chances of getting pregnant without medical help. 
• The presence of fertility problems. 

The chances of their getting pregnant depend upon a number of variables multiplied together. Consider a couple where both the husband and wife have a condition that impairs their fertility. For example, the husband's fertility, based on a reduced sperm count is 50 percent of normal values. His wife ovulates only in 50 percent of cycles; and one of her fallopian tubes is blocked. With three relative infertility factors, their chance of conception is 0.5 (sperm count) X 0.5 (ovulation factor) X 0.5 (tubal factor) = 0.125, or 12.5 percent of normal. Since the chance of conception in normal fertile couples is only 25% in any one cycle, the probability of pregnancy in any given month for this couple without treatment is only 3 percent (0.125 X 25 = 0.03125)! Even if they kept on trying for 5 years, their chance of conceiving on their own would be 60% only. Thus, infertility problems multiply together and magnify the odds against a couple achieving a pregnancy. 

This is why it is important to correct or improve each partner's contributing infertility factors as much as possible in order to maximize the chances of conception.

If infertile couples had 300 years in which to breed, most wives would get pregnant without any treatment at all! Of course, time is at a premium, so the odds need to be improved - and this is where medical treatment comes in.