In the United States, it has been estimated that by 2011, 36% of twin births and 78% of triplet and higher-order births resulted from conception by assisted reproductive technology. Twins
Twins are by far the most common form of multiple births in humans. The U.S. Centers for Disease Control and Prevention report more than 132,000 sets of twins out of 3.9 million births of all kinds each year, about 3.4%, or 1 in 30. Without fertility treatments, the probability is about 1 in 60; with fertility treatments, it can be as high as 20-25%.Dizygotic (fraternal) twins run in families; however, the male does not influence the chances of his partner having twins, even if there are twins in his family. The hyperovulation gene is the cause for having twins and is only a factor for the mother. Although the male does not influence his partners chances of having twins, he could influence his childrens chances for having twins. If a male carries the gene for hyperovulation and has a daughter, she may also have the gene for hyperovulation and then have twins herself. Monozygotic (identical) twins do not run in families. The twinning is random, due to the egg splitting, so all parents have an equal chance of conceiving identical twins. Triplets
Triplets can be either fraternal, identical, or a combination of both. The most common are strictly fraternal triplets, which come from a polyzygotic pregnancy of three eggs.
In May 2021, the Cissé nonuplets were born in Morocco to Halima Cissé, a 25-year-old woman from Mali. As of May 2022, one year since their births, all nine are still living and reportedly in good health. The list of multiple births covers notable examples. Causes and frequency
The frequency of N multiple births from natural pregnancies has been given as approximately 1:89N−1 (Hellins law) and as about 1:80N−1. This gives:

1:89 (= 1.1%) or 1:80 (= 1.25%) for twins
1:892 (= 1:7921, about 0.013%) or 1:802 (= 1:6400) for triplets
1:893 (= approx. 0.000142%, less than 1:700,000) or 1:803 for quadrupletsNorth American dizygotic twinning occurs about once in 83 conceptions, and triplets about once in 8000 conceptions. US figures for 2010 were:
Twins, 132,562, 3.31%
Triplets, 5,503, 0.14%
Quadruplets, 313, 0.0078%
Quintuplets and more, 37, 0.00092%Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as in vitro fertilization (several embryos are often transferred to compensate for lower quality) or fertility drugs (which can cause multiple eggs to mature in one ovulatory cycle). For reasons that are not yet known, the older a woman is, the more likely she is to have a multiple birth naturally. It is theorized that this is due to the higher level of follicle-stimulating hormone that older women sometimes have as their ovaries respond more slowly to FSH stimulation.The number of multiple births has increased over the last decade.