I conceived my first child, a son, with no trouble at all. When he was two, we thought we might have another. A year or so later, when nothing had happened, we saw a fertility specialist, who gave us every test there was. We passed each one, as the consultant put it, "with flying colours". Which left us relieved but also confounded. All I had was a new adjective to add to my diagnosis: Unexplained Secondary Infertility.
The major complication of IVF is the risk of multiple births. This is directly related to the practice of transferring multiple embryos at embryo transfer. Multiple births are related to increased risk of pregnancy loss, obstetrical complications, prematurity, and neonatal morbidity with the potential for long term damage. Strict limits on the number of embryos that may be transferred have been enacted in some countries (e.g. Britain, Belgium) to reduce the risk of high-order multiples (triplets or more), but are not universally followed or accepted. Spontaneous splitting of embryos in the womb after transfer can occur, but this is rare and would lead to identical twins. A double blind, randomised study followed IVF pregnancies that resulted in 73 infants (33 boys and 40 girls) and reported that 8.7% of singleton infants and 54.2% of twins had a birth weight of less than 2,500 grams (5.5 lb).[35]

The diagnosis is one of exclusion — that is, one which is made only after all the existing tests have been performed and their results found to be normal. This is why the frequency of this diagnosis will depend upon how many tests are done by the clinic — the fewer the tests, the more frequent this diagnosis. And the better the tests, the more likely you are getting a diagnosis instead of being told it's "unexplained."

Only 30 percent of patients who receive 100 mg of Clomiphene a day will produce more than three follicles. Patients that produce less than than three follicles have about half the chance of getting pregnant than those that produce greater than three follicles. Patients that receive fertility medications but do not do an insemination have only half the success rates compared to those who do.

In the UK, previous NICE guidelines defined infertility as failure to conceive after regular unprotected sexual intercourse for two years in the absence of known reproductive pathology.[11] Updated NICE guidelines do not include a specific definition, but recommend that "A woman of reproductive age who has not conceived after 1 year of unprotected vaginal sexual intercourse, in the absence of any known cause of infertility, should be offered further clinical assessment and investigation along with her partner, with earlier referral to a specialist if the woman is over 36 years of age."[12]
Problems with your periods or menstrual cycle is a sign of ovulation problems – and if you aren’t ovulating, you won’t get pregnant. Menstrual problems are the most obvious sign of infertility in women – but they don’t necessarily mean you’re infertile. Most women have some type of problem with their period: light flow, heavy flow, clotting, irregularity caused by stress or weight fluctuations, hormonal changes, etc.
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.