In this study, 44% of the women planned for vaginal delivery still ended up having Cesarean section for unplanned reasons such as pregnancy complications. In comparison, it has been estimated that 75% of twin pregnancies in the United States were delivered by Cesarean section in 2008. Also in comparison, the rate of Cesarean section for all pregnancies in the general population varies between 40% and 14%.Fetal position (the way the babies are lying in the womb) usually determines if they are delivered by caesarean section or vaginally. A review of good quality research on this subject found that if the twin that will be born first (i.e. is lowest in the womb) is head down there is no good evidence that caesarean section will be safer than a vaginal birth for the mother or babies.Monoamniotic twins (twins that form after the splitting of a fertilised egg and share the same amniotic fluid sac) are at more risk of complications than twins that have their own sacs. There is also insufficient evidence around whether to deliver the babies early by caesarean section or to wait for labour to start naturally while running checks on the babies’ wellbeing. The birth of this type of twins should therefore be decided with the mother and her family and should take into account the need for good neonatal care services.Cesarean delivery is needed when first twin is in non cephalic presentation or when it is a monoamniotic twin pregnancy.
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.