Six genes found for a woman's likelihood of having fraternal twins

Among the identified genes, four were associated with known roles in female reproduction, affecting the follicle-stimulating hormone (FSH) pathway, particularly relevant to women undergoing fertility treatments. Notably, two genes previously unlinked to twinning risk were also discovered. The genes associate with various aspects of female reproduction but also with body size, confirming that larger women have a higher likelihood of giving birth to twins. The results of the research by the Twinning Genetics Consortium with participation of researchers from the Netherlands Twin Register of the Vrije Universiteit Amsterdam (VU), are published in the scientific journal Human Reproduction.

VU-researcher Hamdi Mbarek observes: “I am involved in many project to identify genes, and we rarely see such a large proportion of findings that have direct meaning”.

Twin-prone women also have earlier onset of menstruation and menopause and more children. Amsterdam UMC professor Niels Lambalk explains: “The genetic findings in this study confirm the well-known close relationship between ovarian ageing normally  associated with less chance of pregnancy but paradoxically more chance of a multiple pregnancy”.

Global difference
While identical twins occur globally at a consistent rate of about 4 per 1000 births, the occurrence of fraternal twinning varies greatly, from 20 per 1000 in Africa to 2 per 1000 in East Asia, with women of European origin falling in between at 8 per 1000. Senior author and VU-professor Dorret Boomsma comments: “When our findings can be combined in future with similar studies in different ancestries, this may finally provide an insight into the old riddle of why so many more twins and triplets are born to mothers in West African countries, and why so few dizygotic twins occur in Asia.

Across the globe, fraternal twinning runs strongly in families; a woman whose mother or sister has had fraternal twins is twice as likely to have them herself. It is the familial component of fraternal twinning which prompted this international group of scientists to focus on these twins and try and find the genes that explain the familial inheritance.

Twins and identical twins
Twins have fascinated us since ancient times. Yet, until recently, their origins have remained a mystery. This is partly because it is only in the last century or so that it has become clear that there are two types of twins, with quite different origins. Identical twins, the ones that capture the public’s imagination and the front covers of magazines, occur when a single egg is fertilized by a single sperm to form a single zygote (hence monozygotic, MZ). At some early stage of development, the embryo spontaneously splits in two, resulting in the birth of two, and sometimes even more, genetically identical babies. VU-researcher Jenny van Dongen studied the epigenetics of MZ twins and explains: “the processes that lead to identical twins are very different from those in nonidentical twins”.

“Nonidentical twins on the other hand, occur when a woman releases two eggs during ovulation and these are fertilised by two different sperm to form two separate zygotes and resulting in the birth of dizygotic (DZ) twins. These twins are just like other brothers and sisters, except they are born at the same time. About 25% of all pairs are boy-boy, about 25% are girl-girl, and 50% are  opposite sex pairs”, says Van Dongen.