Twinning means that two babies are born as a result of one pregnancy. Born babies can be identical (monozygotic) or non-identical (dizygotic, fraternal). Identical twins happen when a single egg cell is fertilized by a single sperm sell and then this fertilized embryo splits into two. Non-identical twins happen when two separate egg cells are fertilized by two sperm cells. The reason for multiple births is complex, both genetic and environmental factors can affect.
[PMID 27406421] Two SNPs Associated With Spontaneous Dizygotic Twinning: Effect Sizes and How We Communicate Them.
[PMID 27132594] Identification of Common Genetic Variants Influencing Spontaneous Dizygotic Twinning and Female Fertility.
[PMID 25882505] Women who deliver twins are more likely to smoke and have high frequencies of specific SNPs: Results from a sample of African-American women who delivered preterm, low birth weight babies.