Continuing Education Activity

Twin births account for approximately 3 percent of live births in the United States. Barring pregnancies that result from assisted reproductive technology, dizygotic twins are far more common than monozygotic twins and account for 70 percent of all twin gestations. Twin gestation in itself is not an adequate obstetric diagnosis. The definition of placental chronicity is essential, as monochorionic twin gestations have unique associated risks that require surveillance. This activity reviews the evaluation and management of twin births and highlights the role of interprofessional team members in collaborating to provide well-coordinated care and enhance outcomes for affected patients and families.

Objectives:

• Summarize the incidence of monozygotic versus dizygotic twins in the United States.
• Describe the contrasting physiology of monozygotic and dizygotic twins.
• Review the risks associated with monochorionic pregnancies.
• Explain the importance of improving coordination amongst the interprofessional team to enhance the management of twin pregnancies.

Introduction

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 thousand births. Except for post-term pregnancy and fetal macrosomia, pregnancy-related risks are exaggerated. Preterm birth is a potential risk associated with twin gestations, with others at risk for fetal growth restriction, congenital anomalies, and abnormal placentation. Other obstetric risks include the increased risk of preeclampsia and gestational diabetes. Twin gestation in itself is not an adequate obstetric diagnosis. The determination of placental chronicity is essential, as monochorionic twin gestations have unique risks associated with them that deserve surveillance.[1]

Etiology

The etiology of twin complications occurs secondary to doubling placental mass and, thus, the associated obstetric complications of preeclampsia and gestational diabetes. Additionally, the uterus may restrict its capacity to distend and permit adequate fetal growth, thus creating a risk for preterm labor. Fetal growth restriction may also increase the risk for preterm labor and birth 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 understanding the possible obstetric risks that can develop and formulate an obstetric care plan.[2]

Epidemiology

The primary factor 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 interventions, including medication use, 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 an increased risk for dizygotic twins.[3][4]

There are racial and ethnic population variations in the prevalence of dizygotic twins, with the risk of 1.3 per 1000 births in Japan to 8 per 1000 births in the United States and Europe. The risk uniquely increases in Nigeria, where dizygotic twins comprise up to 50 per 1000 births. 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) of more than 30 kg/m2 have a high risk for dizygotic twins.[5]

Pathophysiology

In twins, the doubling of placental mass explains the increased risk for complications like preeclampsia and gestational diabetes and an increased risk for placenta previa. A higher incidence of velamentous cord insertion may create additional risks for vasa previa.[6] In monochorionic pregnancies, there are other risks of intraplacental vascular connections between the two fetal circulations. This leads to a risk of twin-twin transfusion syndrome and, later during the pregnancy, twin anemia-polycythemia sequence.[7]

On rare occasions, twin reverse arterial perfusion can occur. This condition leads to an abnormal fetus with the pathophysiology of being "parasitic" to the other twin's circulation.[8] Monochorionic, monoamniotic, and twin gestations can also occur if the embryo splits after seven days. Should pathologic embryonic cleavage occur after 14 days, there is a risk for conjoined 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 carries a high risk for selective fetal growth restriction.[9]

History and Physical

Interval symphyseal fundal height is routinely evaluated in obstetric prenatal care to screen for fetal growth. In twin gestation, this metric is not a reliable reassurance against fetal growth restriction; however, it still has a clinical role. Symphyseal fundal height is typically 4 cm of the expected gestational age. An unexpected increase may indicate polyhydramnios driven by a diverse range of etiologies. In monochorionic twin gestations, this may indicate twin-twin transfusion syndrome, especially if the mother gives a history of thirst and acutely enlarging abdominal girth.[10]

Weight gain surveillance is a critical part of the physical examination. The Institute of Medicine recommends 25 to 54 pounds (11 to 25 kg) 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.[11] Suboptimal maternal weight gain in twin gestations is associated with an increased risk for fetal growth restriction. A routine obstetric examination also includes evaluating blood pressure and the presence of proteinuria. These measures are also critical to the early diagnosis of preeclampsia, a risk that increases in multiple gestation mothers.

Evaluation

The care plan and evaluation for twin gestations depend on the definition of the chorionicity of the pregnancy.[12] A routine, early imaging policy would allow diagnosis of twin gestations and establish the chronicity reliably; however, the American College Obstetricians and Gynecology do not endorse routine ultrasound examination of the abdomen in fetal low-risk pregnancies. Identifying separate placentas is a highly reliable indicator of dichorionic twins; however, a unified placental appearance may represent a single placenta and a monochorionic twin gestation, although not necessarily so. Imaging in the 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. As a peak sign, the intervening gestational membrane insertion into the placenta suggests the presence of a chorion between the two amnions. The imaging study is ambiguous regarding the placement of the intravascular "equator" on the placental chorionic plate that demarcates the 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 be mistaken for a twin-twin transfusion syndrome.[13] When selective fetal growth restriction occurs in conjunction with significant twin-twin transfusion syndrome, the prognosis for twin gestation becomes grim.

Regarding fetal aneuploidy risk assessment, dizygotic twin gestations entail doubling the aneuploidy risk; however, the risk for both affected fetuses is 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 based on 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 three 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 NT-sequential and QUAD screening are available. If genetic amniocentesis is pursued, the risk of procedure-related loss is in the range of 3%. 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 indicate a selective fetal reduction, and mapping of the twin gestation at the time with genetic amniocentesis assumes additional relevance.[14]

Obstetric ultrasound evaluations for twin gestations depend on determining the chorionicity of the gestation. An ultrasound at 16 weeks for a fetal anatomy survey is appropriate in dizygotic twin gestations. Subsequently, it should be done every 3 to 4 weeks for fetal growth and confirmation of fetal anatomy.[15] Multiple fetal anatomy evaluations minimize the risk of missing a fetal anomaly and also allow diagnosis of abnormalities 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.[16]

If fetal growth is appropriate, fetal testing is unnecessary unless there are any alternative concerns. In monochorionic twin gestations, imaging every two 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 the presence of fetal bladders are reassuring signs. Interval fetal anatomy survey and growth assessment occur with every other study. As long as 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 can be assessed every 3 to 4 weeks for twin anemia-polycythemia sequence risk. Twice-weekly fetal testing is recommended for monochorionic twin gestations beginning at 28 to 32 weeks.[12]

Treatment / Management

Maternal support with adequate nutrition is important and may include supplementation with hematinics.[17] Sacroiliac joint dysfunction is a common issue in twin gestations, and prophylactic spine strengthening exercises in the early second trimester can be helpful. Attention to lifestyle and work routine is worthy of review, given the additional physical strain from twin gestation.

Routine obstetric care includes a review every four weeks until 24 weeks and, after that, every two weeks until 32 weeks when weekly evaluations occur. Twin gestations typically conclude around 36 weeks of gestation. Elective delivery in dizygotic twin gestation without complications can be deferred to the 38 to 39-week window. Monochorionic diamniotic twin gestations are best managed expectantly until 34 weeks; after that, delivery becomes an option if there are any significant complications. Typically elective delivery is deferred to the 36 to 37-week window as long as the obstetric course is 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 for cephalic-cephalic presentations and may be considered for cephalic-nonspecific presentations. A breech presentation may indicate an elective cesarean section. Intrapartum obstetric care includes surveillance for dysfunctional labor and risk for postpartum hemorrhage. In the event of premature delivery, appropriate neonatal care should be arranged.[18]

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.[19] 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.[20] Delivery may represent the best obstetric intervention as definitive fetal treatment is unavailable after 26 weeks.

In monochorionic diamniotic twin gestation cases, the demise of a single fetus places the surviving fetus at risk of acute hypotension and resultant a 10 to 15% risk for death. Surviving fetuses may show signs of visceral ischemic injury, including porencephalic brain cysts or disruption of the 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 the 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 preferable if cord entanglement is present.[21]

Differential Diagnosis

• Abdominal tumors complicating pregnancy

• Complicated twin pregnancy

• Hydatidiform mole

• Polyhydramnios

• Single pregnancy

Pearls and Other Issues

Preventing preterm birth, one of the principal complications of twins, needs to be a clinical focus. Unlike women with single 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, 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 understanding based on limited research data. Women with twin gestation who have preterm birth before 34 weeks may be offered supplemental progesterone for subsequent single 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.

Enhancing Healthcare Team Outcomes

Birthing multiple infants add an increased challenge to the interprofessional team in the delivery room. This includes obstetricians, the patient's pediatrician, family clinician, obstetric-trained specialty nurses, neo-natal care nurses, and a clinical pharmacist if medication has a place in the management of twin births. All members of the interprofessional team must communicate openly, and meticulous documentation of all observations, interactions, and interventions in the case is essential. The best outcomes will occur with all interprofessional team members working together and providing delivery care in a coordinated fashion. [Level 5]