Definitions
Multiple gestation births occur when more than one fetus develops during pregnancy.
Zygosity determines if multiple gestation births arise from a single fertilized egg (monozygotic) splitting into two or more embryos or from multiple eggs (dizygotic or polyzygotic) that are fertilized and develop separately.
Since monozygotic twins carry the same genetic material, they are often referred to as identical. Polyzygotic multiples are genetically similar to siblings from separate pregnancies and are often called fraternal.
Classification of the placenta may be useful in determining zygosity. Twin placentation is classified by the placental disk (single, fused, separate), chorion number (monochorionic, dichorionic), and number of amnions (monoamniotic, diamniotic).
Different gender twins are always dizygotic and have a dichorionic placenta. Monochorionic twins are monozygotic and always of the same sex.
Incidence
The incidence of multiple births has steadily increased in developing countries since the early 1980s.
Most of the international and temporal variation in twinning is explained by variation in dizygotic twinning. The prevalence of monozygotic (MZ) twin births has been constant worldwide at 3.5/1000 maternities until recently.
US data from 2010 show an incidence of 33.1 per 1000 total births. The twinning rate rose more than 70% from 1980 to 2009, but the pace of increase has slowed in recent years.
The rate of triplet and higher-order multiple births (triplet/+) in 2010 was 137.6 per 100,000 births. The triplet/+ birth rate rose more than 400% during the 1980s and 1990s, but has declined since 1998.
Estimates of the incidence of twin conceptions are difficult mainly by the unknown number of abortions and early fetal deaths that occur in multiple pregnancies.
Assisted reproductive technologies (ART) have become widely used in the treatment of infertility over the last 30 years. Currently 1.8% of all births in the UK and 1% of US births occur after assisted conception and in some countries such as Denmark the figure is 4.4%. The most well-documented risk of ART is multiple births with 18% of all multiple births resulting from ART. The majority are dizygotic twins, resulting from multiple embryo transfer.
Two-thirds of twins result from dizygosity, while one-third is monozygotic.
Pathophysiology
Per above
Risk factors
Use of ART
Previous or family history of twin pregnancy
Advanced maternal age
Multiparity
Ethnic background (African American>Caucasian>Asian)
Clinical presentation and diagnosis
Multiple gestation pregnancies are usually diagnosed by early ultrasound and may result in several complications of pregnancy and delivery.
Perinatal mortality
The risk of infant death increases with the increasing number of infants in the pregnancy. The perinatal mortality rate is over four times higher in twins and four to nine times higher in triplets.
The increased rate of loss in multiples is explained by an increased risk of both stillbirth and neonatal death. The increased risk of stillbirth is unexplained but is strongly associated with chorionicity, and the neonatal deaths to prematurity. Stillbirth and neonatal mortality rates were significantly higher in monochorionic than dichorionic twins. The increased risk of neonatal death is also associated by the death of the second twin as a result of anoxia arising from complications of vaginal birth but is not apparent among twins delivered by cesarean section.
Twins conceived as a result of infertility treatment tend to be delivered early and to be of lower birthweight than spontaneously conceived twins. According to the Office for National Statistics in the UK in 2010, only 5.6% of singleton births were preterm, compared with more than half (52.7%) of multiple births.
Data from the National Vital Statistics in the United States reported that in 2008, the infant mortality rate for twins was nearly five times, and the rate for triplets was 10 times, the rate for single births. Reliable infant mortality rates could not be computed for quadruplet and higher-order births due to small numbers of infant deaths in those categories.
Intrauterine growth restriction
The growth and development of a twin fetus is affected not only by the same intrauterine factors as a singleton but also by the interaction with the second fetus. Twins compete for nutrition and, a twin may be severely, even lethally, damaged by the cotwin.
Intrauterine growth rates differ between singletons and from about 26 weeks, divergencies are more apparent with increasing gestational age.
The typical pattern of intrauterine growth of twins is similar to that of a growth-retarded singleton in that the weight falls disproportionately more than the occipitofrontal circumference.
52% of twins and 92% of triplets are of low weight compared to 6% of singletons, and as many as 10% of twins and 32% of triplets are of very low birthweight.
Chromosomal abnormalities
A higher prevalence of anomalies has been found among multiple births than among singletons.
The increase is limited to monozygotic twins.
When twins are identified, a scan should be requested at around 12 weeks’ gestational age for a nuchal translucency test to estimate risk of Down syndrome at this stage. Serum measurements used to assess trisomy 21 are not possible in twin pregnancies. Chorionicity is also most accurately determined at this age.
Twin-to-twin transfusion syndrome (TTT)
TTT happens when there is unequal blood exchange from the donor twin to the recipient cotwin through placental vascular shunts.
Growth discordance >20% should raise suspicion of TTT.
The treatment consists of reducing or blocking the blood exchange by amnioreduction, septostomy, laser photocoagulation of placental vessels, or umbilical cord occlusion of the sick twin. Amnioreduction reduces maternal discomfort caused by polyhydramnios and improves fetal circulation by reducing amniotic fluid pressure. Septostomy equalizes amniotic fluid pressure between the two gestational sacs. Laser therapy interrupts intertwin blood shunting and restores two independent circulations. Umbilical cord occlusion is only used when signs of imminent death are present, such as hydrops and cardiac failure (Table 27-1).
Single-twin demise: consequence for the survivor
Three is a multitude of reasons for single-twin demise. These may include the discordant presence of congenital malformations, placental insufficiency and abruption, cord abnormalities such as velamentous cord insertion, infection, and maternal disease such as diabetes and hypertensive disorders of pregnancy.
Single twin demise may result in a poor outcome for both monochorionic and dichorionic surviving twins, with the consequences for the surviving fetus being profound in the monochorionic pregnancy. In utero fetal demise of one twin in a monochorionic pair confers a risk of cotwin demise that is approximately 12%. The risk of cerebral impairment is between 20% and 30%.
Management
Antenatal management
Multiple pregnancies are associated with an increased risk of miscarriage and early fetal loss, structural defects, growth restriction, stillbirth, anaemia, hypertensive disorders, antepartum haemorrhage, instrumental delivery, cesarean section, and postpartum hemorrhage (Table 27-2, Figure 27-1).
Overall maternal mortality associated with multiple births is 2.5 times that for single births.
Close maternal-fetal medicine follow-up is thus required with these high-risk pregnancies.
Intrapartum management
In view of the increased risks of intrauterine death, many obstetricians recommend earlier elective delivery of twins. Many units deliver MCDA twins at 36 weeks and DCDA twins at 38 weeks.
The risks of delivering prematurely are not to be taken lightly. Twins have a sixfold increased risk of mortality compared to singletons. Twins have a high risk of morbidity predominantly associated with preterm birth, resulting in a one in 13 risk of permanent handicap in twins.
Postnatal management
Initial examination of the newborn should include observation for congenital anomalies, deformities from intrauterine crowding, dislocated hips, and cardiac examination. It is useful to examine twins side by side when assessing for discordance.
Admission to the neonatal unit
Criteria for admission of a twin or triplet to a neonatal unit are not different from a singleton and vary according to condition at birth, gestation, weight, level of acuity, and local policy.
The Canadian Neonatal Network described the trends in the rates of admission of preterm twin and triplet infants to neonatal intensive care units (NICUs) across Canada and compared their neonatal outcomes over a 6-year period. They noticed an increment in the proportion of admissions for twins and a reduction in admission of triplets. Overall, mortality and survival without major morbidity improved for both groups compared to previous years.
Risks for morbidities
Neonatal mortality and morbidity is related to severity of the specific condition needing treatment.
Papers reporting on specific neonatal outcomes for twins are scarce; however, twin analyses show that intraventricular hemorrhage, necrotizing enterocolitis, and bronchopulmonary dysplasia may be somewhat familial in origin; therefore, special attention to siblings should be paid when a twin develops any of these conditions.
Special attention should be considered for the severely growth restricted twin, or the recipient of TTT.
Growth restriction has a very high risk of intraventricular hemorrhage and necrotizing enterocolitis.
Nutrition
Unit feeding protocols for multiples should be no different than singletons, based on birthweight, gestational age, and medical stability.
There is strong evidence that nutrition for babies with growth restriction should be provided with expressed breast milk exclusively, ideally from their own mother.
Breast-feeding or bottle supplements should be supported and mothers encouraged to pursue realistically what they would like to achieve in terms of nutrition. Most mothers can breast-feed both twins if carefully prepared antenatal to do so.
Developmental care
Twins should convalesce in neonatal units with developmental care approach.
There is evidence of benefit on provision of a comfortable environment and parental involvement in care.
Skin-to-skin improves short- and long-term outcomes.
Cobedding. Multiples’ sharing a crib is controversial. There is evidence that cobedding improves weight gain and improves cardiorespiratory stability. However, infection control risks and the increased risk of sudden infant death must be considered, especially when nearing discharge. Cobedding should be discouraged postdischarge, when the infants are no longer being monitored.
Individualized care
While sometimes difficult, management of twins should be individualized according to their behavior, maturity, and medical needs.
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