There can be medical causes of SI, says West. "The thyroid is always something we check. Birth can put the thyroid out of kilter." Anwen, a woman in her 40s, tried for five years to have a second child. "I had my daughter when I was 30," she says, "which, at the time, seemed very early. I was the first among all our friends to have a baby." She decided to try for a second when her daughter was three. "But a year went by and nothing happened. I went to the GP and he told me not to worry. 'If you've already given birth, there won't be a problem,' he said. But my daughter turned five and I still wasn't pregnant." Eventually, Anwen persuaded her GP to refer her to a fertility consultant who, after some simple investigations, informed her she had polycystic ovarian syndrome. "He said I'd probably had it all my life and that my daughter was an amazing one-off. I had no idea that anything was wrong." She was given a prescription for the fertility drug Clomid; two months later, she conceived her son.
^ Hozyasz, K (March 2001). "Coeliac disease and problems associated with reproduction". Ginekol Pol. 72 (3): 173–9. PMID 11398587. Coeliac men may have reversible infertility, and as in women, if gastrointestinal symptoms are mild or absent the diagnosis may be missed. It is important to make diagnosis because the giving of gluten free diet may result in conception and favourable outcome of pregnancy.
In humans, infertility is the inability to become pregnant after one year of intercourse without contraception involving a male and female partner. 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%. Male infertility is responsible for 20–30% of infertility cases, while 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.
Sometimes problems getting pregnant for a second or subsequent time are related to a complication that occurred in a prior pregnancy or prior to delivery (damage to the uterus, for instance). But most often, secondary infertility is caused by the same factors that would cause primary infertility — issues like advanced age, obesity, ovulation problems and so on.