Preface:

One in six couples have an unwanted delay in conception. Roughly half of these couples will conceive either spontaneously or with relatively simple advice or treatment. The other half remain subfertile and need more complex treatment, such as in vitro fertilisation and other assisted conception techniques; about half of these will have primary subfertility. Most couples presenting with a fertility problem do not have absolute infertility (that is, no chance of conception), but rather relative subfertility with a reduced chance of conception because of one or more factors in either or both partners. Most couples with subfertility will conceive spontaneously or will be amenable to treatment, so that only 4% remain involuntarily childless. As each couple has a substantial chance of conceiving without treatment, relating the potential benefit of treatment to their chances of conceiving naturally is important to give a realistic appraisal of the added benefit offered by treatment options. Chance of spontaneous conception Conception is most likely to occur in the first month of trying (about a 30% conception rate). The chance then falls steadily to about 5% by the end of the first year. Cumulative conception rates are around 75% after six months, 90% after a year, and 95% at two years. Subfertility is defined as a failure to conceive after one year of unprotected regular sexual intercourse.
It is usually investigated after a year, although for some couples it may be appropriate to start investigations sooner. The likelihood of spontaneous conception is affected by age, previous pregnancy, duration of subfertility, timing of intercourse during the natural cycle, extremes of body mass, and pathology present. A reasonably high spontaneous pregnancy rate still occurs even after the first year of trying. Age A strong association exists between subfertility and increasing female age. The reduction in fertility is greatest in women in their late 30s and early 40s. For women aged 35-39 years the chance of conceiving spontaneously is about half that of women aged 19-26 years. The natural cumulative conception rate in the 35-39 age group is around 60% at one year and 85% at two years. This marked, age related decline in spontaneous conception is also mirrored in the outcome of assisted conception treatment. Recent evidence shows that male fertility also declines with age. Genetic defects in sperm and oocytes that are likely to contribute to impaired gamete function and embryonic development increase with age. The age related decline in female fecundity is caused by a steadily reducing pool of competent oocytes in the ovaries.