Twin births account for approximately 3% of live births and 97% of multiple bouts in the United States. In the absence of assisted reproductive technology, dizygotic twins are far more common than monozygotic twins and account for 70% of all twin gestations. Whereas the instance of dizygotic twins is variable in different populations, the prevalence of monozygotic twinning is globally constant at 3 to 5 per a thousand births. Except for post-term pregnancy and fetal macrosomia, pregnancy-related risks are exaggerated. Preterm birth is a prominent risk associated with twin gestations with others at risk for fetal growth restriction, congenital anomalies, and abnormal placentation. Other obstetric risks that increase include the risk of preeclampsia and gestational diabetes. Twin gestation in itself is not an adequate obstetric diagnosis. Definition of the placental chronicity is essential, as monochorionic twin gestations have unique risks associated with them that deserves surveillance.
The etiology of twin complications occurs secondary to doubling of the placental mass, and thus, the associated obstetric complications of preeclampsia and gestational diabetes. Additionally, the uterus may place restraint in its capacity to distend and permit adequate fetal growth, thus creating risk for preterm labor. Fetal growth restriction may also add to the risk for preterm labor and birth and occurs due to the increased risk of abnormal placentation and function. In monozygotic gestations, there is an additional unique risk of intraplacental vascular connections between the fetal circulations. Thus, the definition of chronicity in twin gestations is critical to developing an understanding of the possible obstetric risks that can develop and for developing an obstetric care plan.
The major factors influencing the occurrence of dizygotic twins is the usage of infertility ovulation induction drugs. Almost one-third of all twin infants born in the United States can be attributed to iatrogenic intervention that includes in vitro fertilization (IVF), ovulation induction, and superovulation with intrauterine insemination. IVF also interestingly increases the risk of embryo cleavage that can generate complications of monozygotic twinning. Maternal age is associated with increased risk for dizygotic twins. There are racial and ethnic population variations with the prevalence of dizygotic twins with the risk of 1.3/1000 births in Japan to 8/1000 births in the United States and Europe. The risk uniquely increases in Nigeria where up to 50/1000 births are dizygotic twins. With parity, there seems to be an increased risk for dizygotic twins even when adjusted for maternal age. Maternal family history for twinning adds to the risk, but the paternal history is non-contributory. Some studies have suggested those patients with a body mass index (BMI) more than 30 Kg/m2 have a high risk for dizygotic twins.
In twins, the doubling of the placental mass explains the increased risk for complications like preeclampsia and gestational diabetes and an increased risk for placenta previa. There is a higher incidence of velamentous cord insertion that may create additional risk for vasa previa. In monochorionic pregnancies, there are additional risks of intraplacental vascular connections between the 2 fetal circulations. This leads to risk for twin-twin transfusion syndrome, and later during the pregnancy, for twin anemia-polycythemia sequence. On rare occasions, twin reverse arterial perfusion can occur. This leads to an anomalous fetus with the pathophysiology of being “parasitic” to the other twin’s circulation. Monochorionic, monoamniotic, twin gestations can also occur if the splitting of the embryo occurs after 7 days. Should pathologic embryonic cleavage occur after 14 days, there is a risk for conjoint twins. Another risk unique to monochorionic twin gestations is an unequal placental mass assignment due to the unequal placement of the vascular "equator" and the intervening membrane. The fetus with a smaller placental mass assigned to it becomes at high risk for selective fetal growth restriction.
Interval symphyseal fundal height is routinely evaluated in obstetric, prenatal care to screen for fetal growth. In twin gestation, such determinations are not adequately a reliable reassurance against fetal growth restriction; however, the still have a clinical role. Symphyseal fundal height is typically 4 cm of the expected for gestational age. An unexpected increase may be indicative of polyhydramnios driven by a diverse range of etiologies and in monochorionic twin gestations may be indicative of twin-twin transfusion syndrome, especially if the history of thirst and acutely enlarging abdominal girth is volunteered. Weight gain surveillance is a critical part of the physical examination. The Institute of Medicine recommends 25 to 54 pounds total weight gain at term for women carrying twins. The lower end of this range is suitable for obese women, the middle of the range is appropriate for overweight women, and the upper end of the range is appropriate for women of normal weight. Suboptimal maternal weight gain in twin gestations is associated with increased risk for fetal growth restriction. A routine obstetric examination also includes evaluation of blood pressure and presence of proteinuria. These are also critical to the early diagnosis of preeclampsia for which the gestations are at high risk.
The care plan and evaluation for twin gestations are dependent on the definition of the chorionicity of the gestation. A policy for routine, early imaging would allow diagnosis of twin gestations and establish the chronicity reliably; however, the American College Obstetricians and Gynecology does not endorse routine ultrasound examination in a population of the abdomen fetal low-risk pregnancies. Identification of separate placentas is a highly reliable indicator of dichorionic twins however it unified appearing placenta may represent a single placenta and a monochorionic twin gestation, although not necessarily so. Imaging in first or early second trimester allows evaluation of the intervening gestational membranes in addition to the placental masses. A thin intervening gestational membrane less than 3 mm with the absence of cyst peak sign is indicative of monochorionic twin gestation. The intervening gestational membrane insertion into the placenta being a peak sign is suggestive of the presence of a chorion between the 2 amnions. The imaging study always remains elusive regarding the placement of the intravascular "equator" on the placental chorionic plate that demarcates assignment of the placental mass to each fetus. An unequal placental mass assignment can lead to selective fetal growth restriction. In such scenarios, the growth-restricted fetus may have severe oligohydramnios within the normal fluid in the adjacent sac and mistaken for a twin-twin transfusion syndrome. When selective fetal growth restriction occurs in conjunction with significant twin-twin transfusion syndrome, the prognosis for the twin gestation becomes grim.
In regard to the fetal aneuploidy risk assessment, dizygotic twin gestations entail doubling of the aneuploidy risk; however, the risk for both affected fetuses becomes theoretical. For monozygotic twin gestations, the aneuploidy risk is congruent, i.e., either both are affected, or both are healthy; thus, the aneuploidy risks for significant gestation for pertinent maternal age can be utilized. When dichorionic diamniotic twin gestation manifests with gender congruence, a distinction between a monozygotic twin gestation with embryonic splitting in the first 3 days cannot be excluded; however, the vast majority can be assumed to be dizygotic. Noninvasive prenatal testing for twin gestations is available. Serum screening of both NT-sequential and QUAD screen are available. If genetic amniocentesis is pursued, the risk of procedure-related loss is in the range of 3% and additional technical expertise is required to map the twin gestation before the procedure, especially if they are believed to be dizygotic with the same gender. Discordant results with one affected fetus may invite option for selective fetal reduction and mapping of the twin gestation at the time with genetic amniocentesis assumes additional sobering relevance.
Obstetric ultrasound evaluations for twin gestations is dependent on the definition of the chorionicity of the gestation. In dizygotic twin gestations, ultrasound at 16 weeks for fetal anatomy survey is appropriate, and after that, it should be done every 3 to 4 weeks for fetal growth and reaffirmation of the fetal anatomy. Multiple fetal anatomy evaluations minimize risk for omissions of fetal anomaly and also allow diagnosis of anomalies that may not present or be available for early diagnosis. Dizygotic twins have an additive risk for congenital anomalies. In monozygotic twins, there is an exaggerated risk for midline anomalies and cardiac anomalies that merit routine fetal echocardiography at 22 weeks. All twin gestations are also at an increased risk for abnormal placentation (placenta previa) and velamentous cord insertions with its attendant risk for vasa previa. Transvaginal scan at 18 weeks with color Doppler mapping allows detection of vasa previa. Intrapartum rupture of the second sac also entails the risk of rupturing an unappreciated gestational vessel; thus the wisdom for seeking recognition of any pulsation in the membranes before their rupture.
As long as a fetal growth is appropriate, fetal testing is not necessary unless an alternative concern indicates it. In monochorionic twin gestations, imaging every 2 weeks is recommended and should commence at 16 weeks. The imaging study provides an opportunity for surveillance of twin-twin transfusion syndrome. Balanced amniotic fluid and presence of fetal bladders are reassuring. Interval fetal anatomy survey and growth assessment occurs with every other study. As long as a fetal growth remains reassuring and findings of twin-twin transfusion syndrome are absent, then fetal middle cerebral artery Doppler peak systolic velocity surveillance for twin anemia-polycythemia sequence is deferred to the third trimester when MCA Doppler's could be assessed every 3 to 4 weeks for twin anemia-polycythemia sequence risk. Twice a week fetal testing is recommended for monochorionic twin gestations beginning at 28 to 32 weeks.
Maternal support with adequate nutrition is important and may include supplementation with hematinics. Sacroiliac joint dysfunction is a common involvement in twin gestations and prophylactic spine strengthening exercises in early second trimester can be helpful. Attention to lifestyle and work routine is worthy of review given the additional physical strain placed upon by twin gestation. Routine obstetric care includes review every 4 weeks until 24 weeks and after that every 2 weeks until 32 weeks when weekly evaluations occur. Twin gestations typically concluded around 36 weeks of gestation. Elective delivery in dizygotic twin gestation that is addressed and without any complications can be deferred to the 38 to 39-week obstetric window. Monochorionic diamniotic twin gestations are best managed expectantly until her to 34 weeks, and after that, delivery becomes an option should significant complications established. Typically elective delivery is deferred to the 36 to 37-week window as long as the obstetric course is entirely stable. The mode of delivery depends on prior obstetric history, current obstetric history, as well as fetal presentation. Vaginal delivery is entirely permissible and indeed recommended cephalic-cephalic presentations and may be considered for cephalic-nonspecific presentations. Breech presentation suppressed delivery device elective cesarean section. Intrapartum obstetric care includes surveillance for dysfunctional labor and risk for postpartum hemorrhage. In event of prematurity, appropriate neonatal care should be arranged.
Unique treatment options for complications of monochorionic dichorionic gestation may be necessary such as laser photoablation of the vascular connections when twin-twin transfusion syndrome occurs. In such scenarios, fetal echocardiography is also warranted to exclude acquired pulmonary stenosis in the donor twin. The occurrence of hydrops fetalis represents an advanced age for twin-twin transfusion syndrome, and as definitive fetal treatment is not available after 26 weeks, delivery may represent the best obstetric intervention. In the context of monochorionic diamniotic twin gestation, the demise of a single fetus places the surviving fetus at risk of acute hypotension and resultant 10% to 15% risk for death. Surviving fetuses may bear stigmata of visceral ischemic injury that may include porencephalic brain cysts or disruption anterior abdominal wall. When a fetal demise occurs, in addition to obstetric challenges, there are additional emotional challenges for the couple, and they deserve supportive care. Fetal treatments are best deferred to tertiary centers. More unusual fetal treatments may include photocoagulation of the umbilical cord to optimize chances for survival of a single fetus when the survival of both the fetuses is not an achievable obstetric goal (such as twin reversed arterial perfusion). In monochorionic monoamniotic twin gestations, severing of the occluded cord may be preferred if cord entanglement is present.
Prevention of preterm birth, one of the principal complication of twins, is a deserving focus. Unlike with women with singleton pregnancies at risk for preterm birth based on prior obstetric history, intramuscular weekly progesterone from 16 to 36 weeks, or cerclage for short or shortening cervix, effectiveness in a twin pregnancy is absent. In other words, neither intramuscular progesterone nor the placement of cervical cerclage reduces the risk of preterm labor in twin pregnancies identified to be at exceptional risk based on abbreviated cervical length or prior history of preterm birth. However, vaginal progesterone may be helpful based on preliminary understandings based on limited research data. Women with twin gestation concluding in preterm birth before 34 weeks may be offered supplemental progesterone for subsequent singleton pregnancies because preterm labor etiologies unique to plural pregnancy (for example, excessive uterine distention) may not be a principal explanation, and as such, recur in future pregnancies.
Birthing multiple infants adds an increased challenge to the interprofessional team in the delivery room. The best outcomes will occur with the clinicians and nurses training together and providing delivery care in a coordinated fashion. [Level V]
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