According to the American Pregnancy Association, there are two ways that a woman can become pregnant with more than one child at a time, either multiple eggs are fertilized at the same time, or one fertilized egg divides into multiple embryos. The chances of becoming pregnant with more than baby is low however. In particular quintuplets are very rare, accounting for less than 41 births in 2011 out of almost four million total births, as noted by Multiples of America.
Multiple babies that come from one divided egg are known as monozygotic multiples. Monozygotic multiples are genetically identical, meaning they are the same gender and look very similar. Monozygotic multiples occur in every three to four out of one thousand births. Whether or not one egg splits into many eggs is entirely random, meaning age, race and genetics have no influence.
Multiple babies born at once can also come from multiple eggs that are all fertilized by different sperm cells. These multiples are known as dizygotic multiples. Because they are made from different eggs and sperm, dizygotic multiples are just like regular siblings in that they are not genetically identical. They can be different genders and look different as well. Women who undergo fertility treatments are more likely to have dizygotic multiples. Other women with an increased risk of having multiple fraternal babies in one pregnancy are those with a high body mass index, those who have had many babies, those who have recently ceased consumption of hormonal birth control, as well as women over the age of 35.
With regard to having quintuplets, any combination of monozygotic and dizygotic multiples can occur. For example, three eggs can be fertilized, and two of those can split into two, producing five embryos.