genes. If you carry two specific genes in your body you are more likely to have fraternal twins. People with these genes tend to respond better to in vitro fertilization. Fraternal twins are are more closely related than identical twins. New research has shown that in vitro fertilization yields more twins than natural conception. The first SNP is FSHB which is involved with the follicle stimulating hormone and the second is the SMAD3. The SMAD3 changes how the ovaries respond to FSH and they believe that this is what creates the fraternal twins. The next research that they plan to do is with women that have SMAD3 and if they are more likely to get pregnant from in vitro fertilization.
I think that this is an amazing discovery. People that have twins now have an answer as to why it happens to them. No one in my family has ever had twins so I know that I do not have these genes.