Background Information on Fertility

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Probability: 'Primary Pregnancy Planners'

Probability: Infertile Couples

Factors Affecting the Probability of Conception

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This section contains important information on the factors that can affect a couple's chances of conceiving. Understanding the the chances of obtaining a pregnancy and how that probability changes with the length of time a couple has attempted to conceive, past pregnancy history, reproductive age, etc. is important in placing patient evaluation and treatment in the proper perspective.


Overview: "Why Haven't We Conceived Yet?"


This is the most commonly asked question by couples seeking medical advice on their inability to conceive. The second most common question is "What can we do about it?" 


It is rare that one specific cause for a couples' failure to achieve a pregnancy is identified - and this is one of the primary reasons why the approach to answering this question should not be one of 'assigning blame'.There are literally 1000's of things which need to happen properly in order for a couple to have a baby - from the successful production of sperm and egg, delivery of the sperm to the female, sperm transport, fertilization of the egg, division of the fertilized egg and formation of an embryo, implantation of the embryo, and gestation eventually ending with the delivery of a baby. Reproduction is an incredibly complex process and the reasons for failure to conceive are complex. Our understanding of human repoduction is incomplete and sometimes a cause is not always identified.


The reasons why couples fail to conceive can be broken down into 3 general causes.



It is imperative that both the male and the female be evaluated for infertility - even if an issue has already been identified with one of the partners.


Before discussing male and female factors, it is important to understand the role of 'chance' in conception and how this helps us to determine the 'probability' of conception.


There are always 2 different possible results when a couple is attempting to conceive: (1) success (conception/fertility) or (2) failure (infertility). There is never a guarantee that a couple will conceive, even if they are perfectly normal - though fortunately most do conceive.  When a pregnancy fails to occur, there does not need to be a cause. A perfectly normal couple may simply fail to have a pregnancy because of 'bad luck' - or 'chance'. Knowing whether bad luck is likely to be the cause of infertility (as opposed to an actual problem with the man or the woman) can only be determined with absolute certainty in a few situations - such as when there is a complete absence of sperm or egg, or when there is complete obstruction of the reproductive tract of one partner (e.g. fallopian tubes are blocked). In most circumstances, we are left to make an educated guess as to whether bad luck is responsible and try and address any factors which may be an impediment to conception.


The natural history of successful couples is the guidepost that is most useful in determining if it's just bad luck or there's likely to be a problem. The bottom line is that the longer a couple has gone without conceiving, the more likely that there is to be a problem. It is generally recommended that couples attempt to achieve a pregnancy for at least one year before seeking medical attention. There are, however, good reasons why some couples should be evaluated sooner - sometimes even before they attempt to achieve a pregnancy.


Natural History of Pregnancy in 'Primary Pregnancy Planners'


'Is there something wrong?' is a frequently asked question of couples who have not been able to conceive. Before answering that question, it is helpful to know what is 'normal' in terms of conception. Not every couple will be able to conceive right away. How long should you wait before being evaluated? Most people would not seek a medical opinion if they hadn't become pregnant after one month of trying - but how about 3 months, or 8 months, or 2 years? We somewhat arbitrarily define couples as infertile if they have not conceived after one year of concerted trying. But just because a couple is labelled 'infertile' does not mean that they won't or can't have children. Unless sterility is present, the answer is a resounding 'NO'. If a man has sperm in his ejaculate and a woman is ovulating and has a patent reproductive tract, there is always a chance of conceiving. The question can then be properly framed as 'What is chance that we will conceive over a given length of time?".


Here is what can be expected in terms of the pregnancy over time in what are called 'first pregnancy planners' - young couples (less than about 30 years of age) with no prior fertility history (no known prior pregnancies) and with no known impediments to a pregnancy. Note that the 'first pregnancy planner' couple will not have had a medical evaluation, so there may be a few who are sterile, but are unaware of their condition. Most couples, however, will be perfectly normal.


Figure: this graph demonstrates the cumulative number of 'first pregnancy planners' who become pregnant over time. The start time is the first month of attempted conception. Each month can be considered one 'try' - since ovulation occurs once a month. Note thate about 6 out of 10 couples will have conceived by 6 months and about 8 out of 10 couples by 1 year (12 months).


The following figure shows this concept another way - by the monthly probability of conception (the rate of conception) instead of the cumulative probability of achieving a pregnancy. Note that the rate of 35% per month is for a female age 25 or less.

There are a couple of important things to note:


  1. Not all couples who eventually become pregnant will conceive within the first year. Between 20%-70% of couples who have not concieved in the first year will concieve in the next 2 years. This is a real spread - it would be nice to know if you were likely to be on the better or worse side of that equation to allow you to make some decisions.
  2. The rate at which couples become pregnant decreases the longer that they failure to conceive - that is, past failure starts identifying couples that are more likely to have an issue from an underlying cause rather than simply by chance.


The other very obvious thing is that the more 'good' attempts at conception a couple has, the more likely they are to conceive. 'Good' attempts means timing intercourse around the anticipated time of ovulation.


Figure: Effect of a male, female or combined fertility problem on the probability of conception over time. If a couple were sterile, the probability of conception would be zero (0) at each time period - i.e. a flat line overlapping the bottom axis.


Factors Which Can Affect the Probability of Conception


If chance is not the primary cause of a couples failure to conceive, what are the possible reasons why they have not conceived? This is a difficult to answer because we do not know all of the reasons for infertility and a substantial number (up to a third) of couples will not have an identifiable factor. Having said that, there are some common reasons why couples fail to conceived:


Male Female Both

Very low sperm concentration

Erectile dysfunction

Advanced reproductive age

Ovulatory/menstrual dysfunction

Tubal disease


Prolonged duration of infertility

 No prior history of successful pregnancy


These issues usually make themselves known by the inability of a couple to conceive. Conversly, just because a couple is able to conceive does not mean that these issues are not present.





This fact becomes most important starting after about 1 year of trying and in situations where the fertility of one or both of the partners is changing for the worse - for example, in a woman who is entering her mid 30's or where there is an identifiable factor such as low sperm count.


While fertility in males and females decline with age starting in the late 20's, it is widely appreciated that fertility in most women starts to decline most rapidly after about 35 years of age. While every woman is different and some can easily conceive well into their 40's, the per-cycle probability of conception diminishes sharply in most women in their late 30's and early 40's. For example, approximately 10% of women age 35 will have difficulty conceiving with the number rising to 30% at age 40 and almost 90% by age 45. Once menopause is reached, a woman can no longer conceive naturally. This is an important consideration, since evaluation should occur sooner than the usual 1 year of attempted conception if a significant decrease in fertility potential is anticpated - for example, in men who will be undergoing chemotherapy or pelvic surgery or women of advanced reproductive age.


There are a large number of reasons for low sperm counts, and for the listed female factors. The major things which can increase the probability of conception include:


  1. Prior history of successful pregnancy, especially within the same couple
  2. Young female reproductive age (less than about 30-35 years old)
  3. Absence of an identifiable impediment to infertility




This study assessed probabilityof conception both in terms of 'next cycle' success but also 'cumulative' success. This is very valuable in deciding if one should perservere with natural conception or start thinking about intrauterine insemination (IUI) or in-vitro fertilization (IVF). Here's a complilation of several of the tables.


    Cumulative Probability of Conception  
  Median intrinsic conception rate <0.05 (first 12 cycles) Over first 12 cycles Over next 12 cycles
Age 25 24 87% 63%
Age 30 18 83% 53%
Age 35 12 73% 39%
Age 40 4 49% 22%




One can use this sort of a calculator to figure out what the cumulative probability of conception would be over any given time frame if one can estimate the rate of conception. This can be done by taking age and duration of infertility data (such as from the paper above) and by making some adjustments based on the specifics of the circumstances (e.g. the effect a specific sperm concentration on conception).


How to use the calculator: 

  • For the probability of success on a single trial, enter the percent of success per attempt - for example: 13% should be input as 0.13
  • For the number of trails, enter the number of months or cycles. For example, for one year input 12
  • For the number of successes, enter 1 (since you're hoping for 1 pregnancy)
  • The answer you are interested in is the Cumulative probability P(X≥x) - which in plain english means the cumulative chance of obtaining a pregnancy over the specified time.


If one inputs a 0.13 probability of success with 12 trials and number of successes 1, the cumulative probabiliy is 0.8119. Translated to infertile couples, this means that a couple with an estimated 13% chance of success per month has an 81% probability of conception over the course of one year.


This method is known as a binomial probability and assumes that the chances of an event don't change over time. While this is not the case for infertility since a woman continues to age, it can give a reasonable estimate for when a woman's fertility is not declining at a very rapid rate. Use with caution when the failure to conceive (or a series of failures) markedly changes the probability of success.


The Probability of Conception in Infertile Couples


The probability of conception in Primary Pregnancy Planners is shown above. How about couples who have been trying unsuccessfully for one year? - note that these are the couples who meet the definition of 'infertility'. Intuition might suggest that the chance of conception is low since a diagnosis of infertility has been assigned. Intuition would largely be incorrect since a significant number of couples who have not conceived within the first year will conceive in the following few years - the fact that the rate of conception is lower notwithstanding. That is, failure to conceive over time is a marker for decreased ability to conceive going forward, but not an absolute inability to conceive. In fact, it is not until about 2-3 years of trying to conceive that the rate of conception decreases to very low levels. The exception is in situations where there is an anticipated or real marked decline in fertility status such as seen with women entering their late 30's and early 40's.


Figure: This is the natural history of couples who have continue with intercourse despite a failure to conceive after 1 year of trying. It can be seen that over the 2 years following an initial year of failure that 2 in 5 couples will achieve a pregnancy on their own. Couples are more likely to conceive if there is no identifiable impediment to pregnancy than those who have an identifiable problem. Date modified from (1) Snick et al. The spontaneous pregnancy prognosis in subfertile couples: the Walcheren primary care study. Human Reproduction 1997 and (2) Collins et al. Prognosis for live birth among untreated infertile couples. Fertility and Sterility 1995.


Therefore, continued attempts at natural intercourse can always be a suitable alternative to ART if the couple understands and accepts the decreased probability of conception. The probability is never 'zero' unless one or both partners is sterile.


On the Web

General Urology Web Sites

CFAS Patient Resources: patient information website for the Canadian Fertility and Andrology Society

Reproductive Facts: patient information website for the American Society for Reproductive Medicine