<Tr> <Th> Frequency </Th> <Td> 113 million (2015) </Td> </Tr> <P> Infertility is the inability of a person, animal or plant to reproduce by natural means . It is usually not the natural state of a healthy adult, except notably among certain eusocial species (mostly haplodiploid insects). </P> <P> In humans, infertility is the inability to become pregnant or carry a pregnancy to full term . There are many causes of infertility, including some that medical intervention can treat . Estimates from 1997 suggest that worldwide about five percent of all heterosexual couples have an unresolved problem with infertility . Many more couples, however, experience involuntary childlessness for at least one year: estimates range from 12% to 28% ." 20 - 30% of infertility cases are due to male infertility, 20 - 35% are due to female infertility, and 25 - 40% are due to combined problems in both parts . In 10 - 20% of cases, no cause is found . The most common cause of female infertility is ovulatory problems which generally manifest themselves by sparse or absent menstrual periods . Male infertility is most commonly due to deficiencies in the semen, and semen quality is used as a surrogate measure of male fecundity . </P> <P> Women who are fertile experience a natural period of fertility before and during ovulation, and they are naturally infertile for the rest of the menstrual cycle . Fertility awareness methods are used to discern when these changes occur by tracking changes in cervical mucus or basal body temperature . </P>

What are the most common causes of infertility in males and females
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