Human reproduction is hopelessly inefficient. The maximum probability of conceiving during a menstrual cycle is only about 40%. One third of conceptions do not result in the delivery of a baby.1^–3 But this inefficient process produces astoundingly good outcomes. The vast majority of continuing pregnancies result in the birth of a healthy human being who will, eventually, pass his or her genes on to the next generation. Miscarriages—clinically detectable pregnancies that fail to progress—are the inevitable byproduct of such a process. They are common and often remain unexplained, even after investigation. They are a source of distress for women and their partners. When a woman has had two or more miscarriages, she is likely to seek professional help, in the hope that a cause and a cure will be found.

Because 10-15% of clinically recognised pregnancies end in miscarriage, and because most women who have one, two, or even three first trimester miscarriages will nevertheless go on to have a successful pregnancy, investigations are usually done only when a woman has recurrent miscarriages.4 …