The U.S. maternal mortality rate for multiple pregnancy is only slightly higher than for singleton. A history of previous dizygotic twins increases the likelihood of subsequent multiple pregnancy 10-fold. Hemorrhage is about 5 times as frequent in multiple as in single pregnancies. The probability of abnormal presentation and of operative delivery and its complications is increased in multiple pregnancy. Premature rupture of the membranes and premature labor, often with a long prodromal phase, are common occurrences in multiple pregnancy. A gravida with a multiple pregnancy has about 5 times the likelihood of having a morbid (febrile, complicated) course as an average patient of the same parity with a single fetus.
Hydramnios is 5 times more frequent in multiple as in singleton pregnancies, principally because of fetal abnormality. “Unlike sex, generally unlike outcome” applies to twins. The greatest loss occurs when both twins are of the same sex, and male pairs succumb more readily than female pairs. Perinatal mortality and morbidity rates are increased in multiple pregnancy, mainly because of preterm delivery and its complications (ie, trauma or asphyxia).
Almost 50% of twins weigh < 2500 g, but the majority of these are of 36 weeks’ gestational age or more. Directly or indirectly, multiple pregnancies are responsible for around 15% of premature births and around 9% of perinatal deaths, a rate 7 times that of single births. Approximately 55% of twins are premature; 80% of perinatal deaths occur in those born before 31 weeks’ gestation, and 93% of deaths are in those with birthweight below 1500 g. The incidence of intrauterine growth restriction is increased in multiple gestation, and multiple gestations account for 17% of infants with intrauterine growth restriction. Congenital malformations and abnormal presentation are more serious in monozygous twins. Preeclampsia-eclampsia, diabetes mellitus, and other disorders may further jeopardize the fetuses.
Efforts to reduce the incidence of prematurity have not met with much success in singletons or in multiple gestations. In order to maximize fetal growth, good nutrition, frequent rest periods, and cessation of smoking are encouraged. Approximately 90% of twins born at greater than 28 weeks survive.
A recent study found that neonatal morbidity is reduced when delivery is accomplished between 37 and 38 weeks and routine induction at that time was recommended.
The comparative occurrence of perinatal death (per 1000) for single and multiple pregnancy are as follows: singletons, 39; twins, 152; triplets, 309; and quadruplets, 509. The rates are proportionately higher for higher-order pregnancies.
The best outlook is for both twins to present by vertex. Twins and other multiple fetuses delivered by spontaneous means do better than those extracted by forceps or after version. Internal podalic version is especially dangerous. Hypoxia and trauma of operative delivery are the primary causes of death of the second twin. Central nervous system disease and hyaline membrane disease are frequently diagnosed in the surviving second twin.
Discordance noted at birth is associated with a slower weight gain during extrauterine life. A twin whose birthweight is less than 20% of that of its partner will not gain as rapidly and may never catch up with the other twin in weight and height. The IQ of the larger monozygotic twin is likely to be higher than that of the smaller twin if the weight difference is more than 300 g at birth. The female twin of a female-male pair who survives cross-transfusion is not sterile (in contrast to the situation encountered in the bovine free-martin).
Concordance of placental examination, clinical comparisons, and hematologic and serologic tests provides presumptive evidence of monozygotic twinning. The total probability of diagnosis of zygosity is over 95% using ABO, MNSs, Rh, Kell, Kidd, Duffy, and Lewis A and B antigens, and approaches 100% using chromosomal analysis.
In comprehensive perinatal care centers, morbidity and mortality rates decrease greatly. In one report of triplets receiving optimal care, the mean weight at delivery was 1779 g and the incidence of neonatal mortality was only 23 in 1000. First twins have about a 3% greater chance of survival than second twins. Breech presentation of the second twin carries higher mortality and morbidity rates.
Revision date: July 6, 2011
Last revised: by David A. Scott, M.D.