MULTIPLE BIRTHS
Saturday, August 16, 2008
Twinning has historically occurred as a demographic variation in one of 80 pregnancies (1.25%). The incidence of twinning in the United States in 1990, however, was 2.3% of pregnancies. A clear distinction should be made between dizygotic (fraternal) and monozygotic (identical) twins. Race, maternal parity, and maternal age affect the incidence only of dizygotic twinning. Drugs that induce ovulation, such as clomiphene citrate and gonadotropins, increase the incidence of dizygotic or polyzygotic twinning quite strikingly. Monozygotic twinning can be viewed as a birth defect; the incidence of malformations is also increased in identical twins and may affect only one of the twins. If a defect is found in one, the other should be examined carefully for lesser degrees of the same defect.
Examination of the placenta can help establish the type of twinning: Two amnionic membranes and two chorionic membranes are found in all cases of dizygotic twins and in one third of monozygotic twins; a single chorionic membrane always indicates monozygotic twins.
Complications of Multiple Births
A. Intrauterine Growth Restriction: There is some degree of intrauterine growth restriction in most multiple pregnancies after 34 weeks. If prenatal care is good, however, the growth restriction is rarely significant. There are two exceptions: The first is the monochorial twin pregnancy in which there is an arteriovenous shunt from one twin’s circulation to that of the other (twin-twin transfusion syndrome). The infant on the venous side becomes plethoric and considerably larger than the smaller anemic twin. Morbidity and mortality rates are considerable in twin-twin transfusion syndrome. Discordance in size— birth weights that are significantly different—can also occur when there are separate placentas. One placenta develops poorly, presumably because of a poor implantation site. In this instance, there is no fetal exchange of blood but there is a striking difference in the growth rates of the two infants.
B. Preterm Delivery: Gestation length tends to be inversely related to the number of fetuses. The prematurity tends to increase the mortality or morbidity of twin pregnancies.
C. Obstetric Complications: Polyhydramnios, pregnancy-induced hypertension, premature rupture of membranes, abnormal fetal presentations, and prolapsed umbilical cord occur more frequently in women with multiple fetuses. In general, most of the complications can be avoided or minimized by good obstetric management. Multiple pregnancy should always be identified prenatally with ultrasound examinations; doing so allows the obstetrician and pediatrician or neonatologist to plan management jointly. The neonatal complications are usually related to prematurity. Prolongation of pregnancy, therefore, leads to a significant reduction in neonatal morbidity.
Follow-up studies of twin pregnancies have yielded conflicting results. In general, the studies do not suggest that twinning has a significant effect on later development, especially if prematurity is excluded as a separate risk factor.
Examination of the placenta can help establish the type of twinning: Two amnionic membranes and two chorionic membranes are found in all cases of dizygotic twins and in one third of monozygotic twins; a single chorionic membrane always indicates monozygotic twins.
Complications of Multiple Births
A. Intrauterine Growth Restriction: There is some degree of intrauterine growth restriction in most multiple pregnancies after 34 weeks. If prenatal care is good, however, the growth restriction is rarely significant. There are two exceptions: The first is the monochorial twin pregnancy in which there is an arteriovenous shunt from one twin’s circulation to that of the other (twin-twin transfusion syndrome). The infant on the venous side becomes plethoric and considerably larger than the smaller anemic twin. Morbidity and mortality rates are considerable in twin-twin transfusion syndrome. Discordance in size— birth weights that are significantly different—can also occur when there are separate placentas. One placenta develops poorly, presumably because of a poor implantation site. In this instance, there is no fetal exchange of blood but there is a striking difference in the growth rates of the two infants.
B. Preterm Delivery: Gestation length tends to be inversely related to the number of fetuses. The prematurity tends to increase the mortality or morbidity of twin pregnancies.
C. Obstetric Complications: Polyhydramnios, pregnancy-induced hypertension, premature rupture of membranes, abnormal fetal presentations, and prolapsed umbilical cord occur more frequently in women with multiple fetuses. In general, most of the complications can be avoided or minimized by good obstetric management. Multiple pregnancy should always be identified prenatally with ultrasound examinations; doing so allows the obstetrician and pediatrician or neonatologist to plan management jointly. The neonatal complications are usually related to prematurity. Prolongation of pregnancy, therefore, leads to a significant reduction in neonatal morbidity.
Follow-up studies of twin pregnancies have yielded conflicting results. In general, the studies do not suggest that twinning has a significant effect on later development, especially if prematurity is excluded as a separate risk factor.
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