Why does one woman spontaneously conceive twins while another never does? That question is central to the research of behavioral geneticist Nikki Hubers. Her study of twin pregnancies provides new insights into the earliest stages of human life and has important social implications for fertility and reproductive care.
Hubers distinguishes between identical and identical twins. Identical twins occur when one fertilized egg splits at an early stage, while in twin twins several eggs mature simultaneously and are fertilized separately. Why this splitting or double ovulation occurs was largely unknown until now. She shows that both forms of twin pregnancies reflect complex biological processes that are crucial to the creation of new life.
Fourteen genetic regions
A key finding is the clear relationship between twin eggs and fertility. Women with a genetic predisposition to having two-egg twins were found, on average, to get pregnant more easily and were more likely to be able to carry riskier pregnancies as well. In the DNA of these women, Hubers identified 14 genetic regions associated with the development of two-egg twins. Many of these genes play a role in the hormonal control of ovulation, particularly through follicle-stimulating hormone and luteinizing hormone.
These genetic insights have broader significance. They explain both why some women have reduced fertility and why other women are more fertile. The results point to a genetic continuum. At one end of the divide are the women with fertility problems and at the other end of the divide are the mothers of two-egg twins. Hubers also examined the effect of these genes in men: a hypothesis now under investigation is that it appears these genes may have an effect with testicular volume.
More than 1.3 million participants
For her research, Hubers used data from large international twin registries, including Vrije Universiteit Amsterdam's Dutch Twin Register, and from population studies such as LifeLines in the northern Netherlands. In total, she analyzed genetic data from more than 1.3 million participants from Europe, Australia and the US. By comparing DNA from mothers of two-egg twins with that of the general population, she was able to uncover patterns that previously remained invisible.
In addition to breakthroughs in knowledge about identical twins, Hubers also mentions the importance of research on identical twins, particularly in the field of epigenetics: changes in gene activity that are not recorded in the DNA itself. More knowledge about these not only contributes to a better understanding of the origins of identical twins, but also to insights into rare diseases and epigenetic regulation in the early years of life. In doing so, twin research touches on fundamental questions about health, disease and development - with impact far beyond the twins themselves.