"We haven't been able to make much progress towards understanding the mechanism in identical twins because of lack of suitable animal models. But, there are two possibilities that we may be able to check. First, discordant ovaries could arise if early post-implantation embryos split unequally during the critical period when progenitor germ cells are forming. Then a subnormal number of follicles might form in one twin. Second, there could be competition in the uterus and the stress might encourage abnormalities in the ways the genes needed to make eggs are expressed in the ovaries of each twin. We are not looking for mutations in the DNA sequence, but gene switches, a topic called epigenetics."

Dr Gosden said that while the great majority of identical and non-identical twins can expect to reach menopause at a similar age to singletons there were clinical implications for a twin with POF whose sibling is still fertile.

"The first option is of ooctye donation for both identical and non-identical twins. For identical twins, in addition, we now have the possibility of ovarian tissue grafting. That might even be considered in some rare cases for non-identical twins if there is a good tissue match.

"This is an important development as, although proportionately rare, the numbers of cases could be substantial in large countries. In the USA, for example, there are 150,000 identical twin pairs in the 20 to 40 age bracket. About 6,000 individuals (4%) will have POF before they are 40 and the data predict a substantial proportion of these will have a sister with a larger ovarian reserve and a likely later menopause."