Have you noticed that there seem to be more twins running around these days? If so, you are right. Since the mid-1970s, the rates of twin births have been increasing steadily and not just in the United States. Historians can track twin birthrates as far back as the 17th century using religious records from across the globe. Modern birth reports show that the rates of twin births started to fall around 1900, reached its low point in the mid-1970s, and have since rebounded. While twinning rates vary across time and place, they tend to follow similar trajectories in most countries.
The two different types of twins, fraternal and identical, are created by two very different mechanisms. Fraternal, dizygotic twins form when a woman releases two eggs in a menstrual cycle, which are then fertilized by two sperm. Fraternal twins are as related as any other siblings from the same parents. In contrast, identical twins, also called monozygotic, are formed when a sperm fertilizes a single egg and the early embryo splits to form two genetically identical embryos. Birthrates of identical twins generally stay constant over time, thus, changes in the rates of fraternal twins cause overall fluctuations in twinning.
What is causing the modern day increase in fraternal twin birthrates? Advances in assisted reproductive technology are commonly cited as the primary cause of increased twinning. However, maternal age and genetics also affect the likelihood that a woman will have twins. Research that occurred prior to the popularization of modern day fertility treatments identified that a woman is four times more likely to have twins at age 35 than at age 15. Since that time, the average age of an American woman’s first child increased from 21.4 years to 25 years. European and Asian women have even higher average first-birth ages that vary from 25.9 to 29.4 years old.
Genetics has long been attributed to playing a role in twinning rates. At the turn of the 20th century, physicians discovered familial clusters of twin pregnancies that eventually lead to the finding that the likelihood of giving birth to fraternal twins could be inherited from either a mother or father. Since fraternal twins are born when a woman releases two eggs in one menstrual cycle, only women can express this trait. More recently, research in genetics identified a variety of genes in humans and other animals that may contribute to fraternal twinning, including ones called bone morphogenic protein 15 and growth differentiation factor 9.
Many of these genes are involved in the development of immature eggs, called oocytes. Altered activity of the resulting proteins may cause women to mature multiple eggs per menstrual cycle and have predispositions to fraternal twinning. Interestingly, environmental factors may also affect twin rates. Studies of modern and historical records show that more twins are conceived during the summer and autumn months, which may be attributed to changes in day light or food supply. Conflicting research data also indicate that smoking, folic acid intake, and recent oral contraceptive use may also affect the likelihood of having fraternal twins. Hopefully, further research will determine if suspected environmental factors of twinning are genuine and if they affect the same biological pathways as genetic causes.