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Naturally, twins occur in about one in 250 pregnancies, triplets in about one in 10,000 pregnancies, and quadruplets in about one in 700,000 pregnancies. After 6 to 8 weeks, ultrasound should provide an accurate assessment of the number of fetuses. An “appearing twin” may be found after the 5th week in nearly 10% of non-identical twin or multiple pregnancies and in over 80% of cases of identical twins. Ultrasound examinations performed early in the 5th week of pregnancy occasionally may fail to identify all fetuses. When a fetus is lost in the first trimester, the remaining fetus or fetuses generally continue to develop normally, although vaginal bleeding may occur. A fetal loss rate of 40% may occur in pregnancies with triplets or more. Spontaneous losses are even higher in triplet and quadruplet pregnancies. Sometimes, very early in a twin pregnancy, one of the fetuses “disappears.” This is referred to as the “vanishing twin syndrome.” Even after ultrasound has shown heart movement in twins, spontaneous loss of one of the fetuses occurs in up to 20% of twin pregnancies.
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Most of the time, this is the type of twinning that occurs from assisted reproduction procedures. The two embryos that result are dizygotic, not genetically identical, and can be the same or different sex. Non-identical twins occur when two separate eggs are each fertilized by a separate sperm. In rare cases, identical twins may be in the same amniotic sac. Depending on when the division occurs, identical twins may have separate placentas and gestational sacs, or they may share a single placenta but have separate sacs. Each embryo is monozygotic, genetically identical, and both will be the same sex. Identical twins occur when a single embryo, created by the union of a sperm and an egg, divides into two embryos. You may know someone who has twins, but do you know how twins occur and how they develop? There are two types of twins: identical and fraternal (non-identical). If you are at risk for a multiple pregnancy, this booklet will help you learn how and why multiple pregnancies occur and the unique issues associated with carrying and delivering a multiple pregnancy. Although major medical advances have improved the outcomes of multiple births, multiple births still are associated with significant medical risks and complications for the mother and children. Also, more women are waiting until later in life to attempt pregnancy, and older women are more likely than younger women to get pregnant with multiples, especially with fertility treatment. However, since the first publication in 1998 of the American Society for Reproductive Medicine’s (ASRM’s) Guidelines on Number of Embryos Transferred, the number of treatment-related pregnancies with triplets or more has decreased dramatically.
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There are more multiple births today in part because more women are receiving infertility treatment, which carries a risk of multiple pregnancy. According to the US Department of Health and Human Services, the twin birth rate has increased by over 75% since 1980, and triplet, quadruplet, and high-order multiple births have increased at an even higher rate.
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Multiple births are much more common today than they were in the past. Multiple Pregnancy and Birth: Twins, Triplets, and High Order Multiples (booklet)