A similar pair of twin sisters are Helen (of Troy) and Clytemnestra (who are also sisters of Castor and Polydeuces). The theme occurs in other mythologies as well, and is called superfecundation. In certain medieval European chivalric romances, such as Marie de Frances Le Fresne, a woman cites a multiple birth (often to a lower-class woman) as proof of adultery on her part; while this may reflect a widespread belief, it is invariably treated as malicious slander, to be justly punished by the accuser having a multiple birth of her own, and the events of the romance are triggered by her attempt to hide one or more of the children. A similar effect occurs in the Knight of the Swan romance, in the Beatrix variants of the Swan-Children; her taunt is punished by giving birth to seven children at once, and her wicked mother-in-law returns her taunt before exposing the children. Ethics of multiple births
Medically assisted procreation
In vitro fertilization: In vitro fertilization (IVF) was first successfully completed in the 1970s as a form of assisted reproductive technology. Out of all the assisted reproductive technology available that is currently in practice, in vitro fertilization has the highest chance of producing multiple offspring. Per each female egg, IVF currently has a 60-70% chance of conceiving. Fertilization is made possible by administering a fertility drug to the eggs or by directly injecting semen into the eggs. There is an increased chance for women over the age of 35 to have multiple births.
In very rare cases, there may be conjoined twins, possibly impairing function of internal organs. Mortality rate (stillbirth)
Multiples are also known to have a higher mortality rate. It is more common for multiple births to be stillborn, while for singletons the risk is not as high. A literary review on multiple pregnancies shows a study done on one set each of septuplets and octuplets, two sets of sextuplets, 8 sets of quintuplets, 17 sets of quadruplets, and 228 sets of triplets. By doing this study, Hammond found that the mean gestational age (how many weeks when birthed) at birth was 33.4 weeks for triplets and 31 weeks for quadruplets. This shows that stillbirth happens usually 3–5 weeks before the woman reaches full term and also that for sextuplets or higher it almost always ends in death of the fetuses. Though multiples are at a greater risk of being stillborn, there is inconclusive evidence whether the actual mortality rate is higher in multiples than in singletons. Prevention in IVF
Today many multiple pregnancies are the result of in vitro fertilisation (IVF). In a 1997 study of 2,173 embryo transfers performed as part of in vitro fertilisation (IVF), 34% were successfully delivered pregnancies. The overall multiple pregnancy rate was 31.3% (24.7% twins, 5.8% triplets, and .08% quadruplets). Because IVFs are producing more multiples, a number of efforts are being made to reduce the risk of multiple births- specifically triplets or more. Medical practitioners are doing this by limiting the number of embryos per embryo transfer to one or two.