The twin birth rate hit a new high in 2014 at 33.9 twins per 1,000 births.1 This has been attributed to an older maternal age at conception, which naturally increases the rate of twinning, and an increased use of assisted reproductive technology. The triplet and higher-order multiple birth rate peaked in 1998 at 193.5 births per 100,000 births. Since then, the rate of higher-order multiple births has decreased by 40 percent (113.5 per 100,000 births in 2014) likely due to a change in assisted reproductive technology procedures 1
Fetal and Infant Morbidity and Mortality
Multiple gestations are inherently at increased risk for maternal and neonatal morbidity and mortality and are associated with significantly higher healthcare costs. The rate of preterm labor and delivery, cesarean delivery, fetal growth and amniotic fluid abnormalities, structural abnormalities, preeclampsia, gestational diabetes, stillbirth and neonatal morbidity/mortality increases as the number of fetuses increase.
One of the greatest complications is preterm birth or delivery prior to 37 weeks gestation (spontaneous or iatrogenic). This occurs in up to 60 percent of twin gestations.2 The risk for long-term morbidity is inversely associated with gestational age at the time of delivery. Preterm infants are at increased risk for breathing and feeding difficulties, developmental delay, cerebral palsy, necrotizing enterocolitis, intraventricular hemorrhage, and vision and hearing impairment.
The risk for stillbirth is also significantly increased. In 2009, the associated stillbirth rate was 12 per 1,000 twin births and 31 per 1,000 triplet and higher-order multiple births compared to five per 1,000 singleton births.3,4
In an effort to improve outcomes of twin gestations, ultrasound assessment of chorionicity, fetal anatomy, biometry, Doppler velocimetry and amniotic fluid volume has become standard practice. Also, consultation with a tertiary care center should be considered in all twin gestations.
The First Trimester Ultrasound
First trimester ultrasound in twin gestations is key to establishing dating and to determining chorionicity and amnionicity. This information dictates the frequency of the need for follow-up imaging. During this exam, the membrane thickness at the site of the insertion of the amniotic membrane into the placenta is evaluated as well as the presence or absence of the “lambda sign.” (See Image 1.)
In a dichorionic diamniotic (DCDA) gestation, each fetus has its own placenta and amniotic sac, and a lambda sign should be present at the insertion of the amnion into the placenta. If the fetuses are sharing a placenta – or monochorionic-diamniotic twins – the dividing membrane will be thin (<2mm) and a “T sign” will be present. (See Image 2.) A monochorionic monoamniotic (MCMA) gestation will share the same placenta and amniotic sac. This occurs in approximately 1 in 30,000 to 1 in 60,000 pregnancies.
Once the type of twinning is determined, then labeling of the twin fetuses should be performed in a consistent and reliable way. This allows consistency in follow-up imaging and is especially important in cases with fetal structural concerns or growth and amniotic fluid abnormalities. Typically, twins are identified by mapping their cord insertion into the placenta and/or labeling “left or right” and “upper or lower.”
If there is confusion about the chorionicity, amnionicity or dating, then referral to a tertiary care center is recommended. It is also recommended in all cases of monochorionic twinning.
Aneuploidy screening in multifetal gestations is not as sensitive as in singleton gestations. This is due to analytes from both fetuses being averaged together, which can potentially mask abnormal values. Nuchal translucency screening in the first trimester with the addition of biochemical testing is the current standard for twin gestations. Noninvasive prenatal testing, which evaluates fetal cell free DNA in maternal serum, is a potential screening tool for aneuploidy, but more information is needed before this testing becomes standard of care.9
Second and Third Trimester Follow Up
Dichorionic Diamniotic Gestations
Patients with an uncomplicated dichorionic diamniotic twin gestation should have a targeted anatomical survey at 18- to 20-week gestation. Thereafter, monthly follow-up for evaluation of interval growth and fluid assessment is recommended. Antenatal testing is not recommended in uncomplicated dichorionic twin gestations.
If the fetal growth remains concordant and there are no other comorbidities, then delivery is usually recommended at 37 to 38 weeks gestation. This is supported by a 2016 systematic review of timing of delivery in uncomplicated dichorionic twin gestations. This study found that the risk of stillbirth was equivalent to the rate of neonatal death at 38-39 weeks, and therefore delivery during the 37-38 week window minimizes the risk of perinatal deaths near term.5
Monochorionic Diamniotic Gestations
Monochorionic twins have more complications, require more frequent follow up and should be managed in a tertiary care center. Monochorionic diamniotic twins develop twin-twin transfusion syndrome (TTTS) in 9 percent to 15 percent of pregnancies6,7 while monochorionic monoamniotic develop TTTS in 6 percent of pregnancies.8 Both are also at risk for developing twin anemia polycythemia sequence (TAPS). This is a condition in which the hematocrits differ significantly without development of oligohydramnios and polyhydramnios. Monochorionic gestations are inherently at higher risk for anomalies and stillbirth.
Uncomplicated monochorionic pregnancies should have imaging every 2 weeks starting in the second trimester to assess for TTTS and TAPS. If these complications occur, then referral to a fetal therapy center for laser ablation of the communicating placental vessels is recommended. If complications do not arise, then surveillance at 2 week intervals should continue until antenatal testing is instituted around 32 weeks gestation.
If the pregnancy progresses without complication, then delivery is recommended at 36 to 37 weeks gestation. This is due to the increased risk of stillbirth in monochorionic gestations. This is supported by the 2016 systematic review on timing of uncomplicated monochorionic diamniotic twin gestations, which found a trend toward an increased rate of stillbirths than neonatal deaths beyond the 36th week gestation.8 There are no randomized controlled trials on this issue to guide management.
Monochorionic Monoamniotic Gestations
Monochorionic monoamniotic twin gestations occur in approximately 1 percent of twin gestations and are complicated by an increased risk for stillbirth due to entanglement of the umbilical cords, structural abnormalities, TTTS & TRAPS. These pregnancies are followed closely with ultrasound, and ultimately patients are admitted in the early third trimester for intensive surveillance. Mono-mono gestations typically are delivered by 32-34 weeks gestation by cesarean delivery.
Mode of Delivery
Route of delivery for twins is typically individualized and is based on many factors. More than 60 percent of twin births are by cesarean delivery.9 Factors such as presentation of each twin, chorionicicty/amnionicity, estimated fetal weights, structural concerns and maternal co-morbidities are considered when planning mode of delivery.
Take Home Points
Twin gestations are inherently at increased risk for maternal and neonatal morbidity. Consultation with a tertiary care center or perinatologist early in gestation is helpful to determine pregnancy risks and to establish a management plan. A multidisciplinary team approach is critical to achieving a healthy outcome for mom and baby.
- Hamilton BE, Martin JA, Osterman MJ, Curtin SA, Mathews TJ. Births: final data for 2014. Natl Vital Statistics Report 2015; 64:1-64.
- Martin JA, Hamilton BE, Ventura SJ Osterman MJ, Kirmeyer S, Mathews TJ, et al. Births:final data for 2009 Natl Vital Statistics Report 2011;60:1-70.
- Tucker J, McGuire W. Epidemiology of preterm birth. BMJ 2004; 329: 675–
- Garne E, Andersen HJ. The impact of multiple pregnancies and malformations on perinatal mortality. JPerinatMed2004; 32: 215–8.
- Cheong-See F, Schuit E, Arroyo-Manzano D, et al. Prospective risk of stillbirth and neonatal complications in twin pregnancies: systematic review and meta-analysis. BMJ 2016; 354:i4353.
- Sebire NJ, Snijders RJ, Hughes K, et al. The hidden mortality of monochorionic twin pregnancies. Br J Obstet Gynaecol 1997; 104:1203.
- Lewi L, Jani J, Boes AS, et al. The natural history of monochorionic twins and the role of prenatal ultrasound scan. Ultrasound Obstet Gynecol 2007; 30:401.
- Steer P. Perinatal death in twins. BMJ 2007; 334:545.
- Practice Bulletin No. 169: Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies. Obstet Gynecol 2016; 128:e131.