**2. Pathophysiology of high-order pregnancy**

A multiple gestation can either occur by a single fertilized ovum splitting (monozygotic), or by the fertilization of more than one ova at the same time (multizygotic), or even by the combination of the above. For monozygotic pregnancies, the timing of the fertilized ovum splitting dictates the pregnancy chorionicity. When the splitting occurs soon after the fertilization, it results in a diamniotic-dichorionic pregnancy. A diamniotic-monochorionic pregnancy occurs when the fertilized egg splits between the third and the eighth day after fertilization and a monoamniotic-monochorionic when the splitting occurs between the ninth and twelfth day. Conjoined twins are formed when the twinning process occurs after the formation of the yolk sac. A high-order pregnancy can harbor any of the above listed combinations [18]. Fetuses that result from the splitting of the same fertilized ovum are called monozygotic and the babies have the same genetic profile, physical characteristics, and sex, while fetuses that result from the fertilization of more eggs have different genetic background and can be of different sex.

In 1896, the Lyon quintuplets died within 15 days from birth while the Dionne quintuplets born in 1934 were the first to survive through infancy [2]. Nowadays, even with the galloping improvement in perinatal care, a high-order pregnancy still represents a challenge where

Dionys Hellin still since 1895 was trying to predict the occurrence of multiples in nature [3]. The "Hellin-Zeleny's rule" (after the publication of Zeleny's work in 1921 [4]) was a simple way to predict the occurrence of multiple births. Among others, Peller in 1946 [5] and Allen in 1960 [6] proposed new versions or improvements of this rule with various imitations [7, 8]. The incidence of multiple births has changed over time, presenting a decline until the late 1970s and an increase thereafter [9]. It is estimated that in the United States in 1980 there were 37 triplets or higher order births per 100,000 births, rising up to 1935 per 100,000 births in 1998 [10, 11]. Quadruplet and quintuplet or higher births raised from 229 and 40 in 1989 to 627 and 79, respectively, in 1998. Since 1999, a decline is observed, and in 2016, there were 1014 per 100,000 triplets or higher order births, 217 quadruplet and 31 quintuplet and other higher order births [12]. In France, the increase of triplets or high-order pregnancies reached 310% and in England and Wales 430% [13]. There is a difficulty to estimate accurately how many of these high-order pregnancies are a result of natural conception, estimates put this number around 20% [14].

Modern fertility treatments are the most important reason for the increase in multifetal pregnancies. Medical societies around the world have recognized the problem and have published guidelines in order to minimize the frequency of high-order pregnancies resulting from fertility treatment. The economic side of the matter is also of great importance. In 1996, there was a report concluding that the cost per woman who delivered singletons or twins was approximately \$39,000, while the cost per woman who delivered triplets or quadruplets was up to \$340,000 [15]. In 2006, the annual cost of each preterm neonate in the USA was estimated around \$51,600 [16]. Since 90% of high-order pregnancies are born prematurely, considering that the average gestational age at delivery for quadruplets is 29.5 weeks and for quintuplets 29 weeks [17], it is easy to understand the impact of multifetal pregnancies to a tight health system budget. Aside the financial cost to the health system, a high-order pregnancy can wreck any family financial planning. The average costs per child is about \$233,680 in the USA (for a child born in 2015, data published by the U.S. Department of Agriculture), without the college education. The birth of four, five, or more children can make the cost for the family unbearable.

Together with the progress in infertility treatment, a lot of progress is made in prenatal and neonatal care. Ultrasound has changed maternity care with the early diagnosis of fetal conditions. New techniques, such as fetal reduction, have made possible a favorable outcome from a pregnancy with normally poor outcome. Nevertheless, a high-order pregnancy still remains a challenge for both the clinician and the mother, not only during pregnancy but also after birth.

A multiple gestation can either occur by a single fertilized ovum splitting (monozygotic), or by the fertilization of more than one ova at the same time (multizygotic), or even by the

**2. Pathophysiology of high-order pregnancy**

outcome is not always favorable.

160 Multiple Pregnancy - New Challenges

A multiple pregnancy can occur as a consequence of a single ovum fertilization. Liu in 2010 [19] reported a case of a monozygotic quadruplet pregnancy as a result of IVF, although the pregnancy ended in the ninth week of gestation due to chromosomal abnormalities. Nnadi in 2013 [20] described a case of monozygotic quadruplet pregnancy conceived naturally that ended in an elective cesarean section of a set of monochorionic tetra-amniotic quadruplets at 37 weeks of gestation. Neubecker in 1962 [21] and Rau in 1940 [22] reported two cases of monozygotic quintuplet pregnancies, indicating the possibility of a quadruplet and quintuplet pregnancy arising from a single ovum.

The term superfecundation is used to denote the fertilization of a second or more ova within hours or days after a first fertilization, which can result in a multiple pregnancy. The two fertilizations can be either due to differed sexual intercourses or as a consequence to sexual intercourse in a context of in vitro fertilization and embryo transfer. Peigné in 2011 [23] reported a case of a multiple pregnancy as a result of intercourse 1 day after the oocyte pick-up, while Milki in 2001 [24] reported a case of multifetal pregnancy occurring after sexual intercourse 5 days prior to oocyte retrieval. Superfecundation raises the issue of paternity among separately fertilized ova. Wenk et al. in 1992 [25] reviewed a parentage test database of 39,000 records of parents involved in paternity suits and reported the percentage of heteropaternal superfecundation among dizygotic twins to be as high as 2.4%. In contrast to superfecundation, the term superfetation is used to describe a multiple pregnancy resulting after ovulation, and fertilization occurs during an established pregnancy. Superfetation has not yet been proven possible in humans, although it exists in animals [26].
