Fraternal twins are conceived when two eggs are fertilized by different sperm cells in the same menstrual cycle. They are twice as common as identical twins, which are conceived when one egg is fertilized by one sperm cell. The fertilized egg splits into two, forming two embryos in the uterus.

There are several factors that may increase your chances of having twins including age, ethnicity, family history and the use of fertility treatments, but how common is it? Your chances of having double-the-fun may be greater than your think.

According to a report by the CDC, the twin birth rate in 2018 was 32.6 twins per 1,000 births. The rate then rose 76% from 1980 to 2009 (from 18.9 to 33.2 per 1,000), was generally stable from 2009 through 2012, and then rose in 2013 and 2014. The 2014 rate of 33.9 was the highest ever reported. Also, the triplet and higher-order multiple birth rate was 93.0 per 100,000 births in 2018. and down 52% from the peak in 1998. From 2019 to 2020, the number of twin births in the United States fell by 7%.

Why the rise and fall in twin rates?

Medically-assisted reproduction, which includes in vitro fertilization (IVF), may largely explain this rise, with the transfer of more than one embryo during the procedure. Also, more women are having children at a later age, which may also partially explain the rise of twin births, because older women are more likely to conceive twins. But why the drop between 2019 and 2020? Researchers believe this is a result of the covid pandemic, and attribute the change to a dramatic decline in fertility treatments at the beginning of the pandemic. IVF and other assistive reproductive technology are associated with a higher number of multiple births. See the study by the CDC.