Getting it right

Pregnancy is a complicated process that depends on many factors:

  • The production of healthy sperm by the man and healthy eggs by the woman
  • Unblocked fallopian tubes that allow the sperm to reach the egg
  • The sperm’s ability to fertilise the egg when they meet
  • A genetically healthy embryo
  • The ability of the embryo to implant in the uterus.

Repeatedly encountering difficulty at any of these steps can lead to infertility. The reality is that human reproduction is a fairly inefficient process. For average fertile couples, the chance of fertilisation from any particular meeting of egg and sperm is about 80%, but by the time of the expected menstrual period, roughly half of these pregnancies will have already failed because the early embryos have not developed. 

Many of these lost embryos are fundamentally abnormal and are unable to survive. In fact, the menstrual period might not even be delayed and many couples do not realise that an early pregnancy has been lost. Even if a menstrual period is missed, in a quarter or more of pregnancies, the embryo still fails later. 

Various clinical studies have calculated the chance of pregnancy among population groups who do not use contraception, and indicate that the probability of a live birth after exposure of the egg to sperm in any one month averages about 1 in 5, or a 20% chance.

As you can see by the graph below, by age 36 an average woman’s chances of conceiving per month is decreased by nearly half. The downward slope continues until by age 45 the average natural fertility rate per month is approximately 1%.

Chances Of Conceiving Pm

Unexplained infertility

The cause of infertility is not always apparent. In many cases, both partners can appear to be fine, but still there’s no pregnancy. This is known as unexplained infertility.

If someone has an obvious, serious reproductive condition, it is often easier to diagnose and treat the probable cause of their infertility. Ironically, mild problems may be harder to identify and resolve, particularly if there are several minor factors that combine to influence fertility.

Degrees of fertility

Infertility can be ‘complete’ (also called ‘sterility’), which means you absolutely cannot get pregnant without help. Or it can be ‘incomplete’, also called ‘subfertility’, which means that there is no absolute barrier but the odds are reduced compared with normal and getting pregnant will take more time. 

An average fertile couple in their 20s having regular sex would have a monthly chance of pregnancy of about 20%. Pregnancy would be expected within 4-6 months of trying. When a year has passed without successful conception the monthly chance of pregnancy will be much lower than this. This is the point at which couples are advised to consider seeking help.

There may not be a major reproductive condition affecting fertility levels. Even minor disturbances, especially in combination, can cause the monthly chance of pregnancy to be substantially reduced.

To illustrate the effect of multiple, minor factors in infertility, a model can be used to estimate the chance of pregnancy per month and the amount of time it will, on average, take to get pregnant:


No. of mild factorsMonthly chanceEstimated time to a pregnancy
0 20% 4 months
1 5% 2 years
2 1% 7 years
3 0.2% 40 years


It is easy to see that it doesn’t take much to reduce the chance of pregnancy quite significantly. Just a slightly low sperm count and mild endometriosis (both very common conditions) are enough to extend the expected time to conception to 7 years.

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