This is especially prevalent after the age of 35. Some fraternal twins can even be conceived on different days in the reproductive cycle. On rare occasions superfetation may occur when a second egg is fertilized in a following cycle, resulting in two babies due at different times. It rarely happens because of the circulating hormones which are initiated with the beginning of pregnancy. Less twins are born to Asians than any other race yet the Japanese are more likely to have identical twins rather than fraternal twins.
Twins are more likely to occur in larger families as well.
This also goes along with the increased twinning rate in older mothers for those are the ones more likely to have more children. New fertility treatments are greatly increasing the twin rate beyond what it was in previous years. Today, at least 20 percent of pregnancies conceived with one medication called Pergonal, will end up having twins. Fraternal twins are also the hereditary kind. It is one of the most frequently asked questions of twin parents.