Infertility Monthly Probability
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The monthly probability of pregnancy
One important concept for all couples who are trying to achieve pregnancy to understand is the monthly chance of a conception occurring. Contrary to most expectations, this is not very high and only 15 - 20% at best for a healthy young couple. This is the reason why it takes many couples several months to conceive after ceasing contraception. Most of these pregnancies occur in the first 6 months of trying.
If the couple have a problem interfering with fertility their monthly chance of conception (fecundability) may be reduced to <10%. If several factors are at play then conception rates may be as low as 1% per month. That is to say couples with even severe problems may not be absolutely infertile so much as subfertile. It may take them longer to get pregnant than the idealised couple with 100% fertility or they may never conceive.
Rather than exhaustively pursuing fertility tests and looking for the cause for infertility most specialists use the monthly chance of fertility as an indication of the point in time when assistance to the reproductive process is required. It is possible to estimate the monthly probability of pregnancy by considering
• the length of time the couple have been trying
• female age
• Lifestyle factors: BMI and smoking
• Primary (no previous pregnancies) versus secondary infertility which has a better prognosis
• known infertility factors.
If the monthly chance of pregnancy is below 5%, then it is appropriate to consider one of the technologies which assist reproduction.
As women age their fertility declines with this reduction in the probability of pregnancy usually commencing in their mid 30s and becoming much more rapid in their early 40s. Very few women will conceive their first baby beyond the age of 43. Monthly fecundity for a woman in her early 40s without other infertility factors is only 2-3%. The reason women become less fertile as they age is because their egg quality declines. Women are born with a finite number of eggs that they use over their reproductive lifetime. As they age their eggs age with them. In your 40s egg quality and quantity has reached the limits of reproductive capacity.
Contrast this with men who remain fertile up to any age if sexual function is maintained. Although sperm quality may decline generally fertility remains. This is because sperm production does not commence until puberty and does not stop until death. Spermatogenesis takes 80 days to occur within the testicles and hence sperm is only ever a few months old as opposed to decades old for eggs.
Older couples should be offered more prompt evaluation and access to treatment as delays in referral can impact upon the likelihood of pregnancy being achievable with intervention.alist.