1. Killion, Molly M. MS, RN, CNS

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The incidence of twins increased 76% from 1980 to 2009, mostly attributed to higher use of assisted reproductive technology and advanced maternal age at conception (American College of Obstetricians and Gynecologists [ACOG], 2018). Zygosity, or the distinction of identical being from one zygote splitting into two (monozygotic twinning) and fraternal being two separate zygotes from simultaneous conception (dizygotic twinning) is well understood; however, the importance of determining chorionicity and the associated issues is not. Chorionicity describes the type of placentation including the formation of the amnion (closest to the fetus) and chorion (closest to the placenta) layers of the amniotic sac (De Paepe, 2019). Dizygotic twins account for 70% of all twins and will always have separate amnions, chorions, and feto-placental circulations, even if their placentas are fused (De Paepe). Monozygotic twins, though only 30% of twins, can have many more iterations including shared amnions, chorions, and placental circulation that can then lead to complications.


Understanding chorionicity must be established early in gestation using ultrasonography in the first trimester or early second trimester to determine the plan for follow-up screening and timing of birth planning (ACOG, 2018).


* Dichorionic/Diamniotic: results with early division of the zygote within 3 days of fertilization; 70% of monozygotic twins (De Paepe, 2019). In the absence of complications*, birth at 38 weeks is recommended (ACOG, 2018); however, there is argument that birth at 37 weeks is safer (Malone & D'Alton, 2019).


* Monochorionic/Diamniotic: division in the blastocyst stage at 3 to 9 days after fertilization; 25% of monozygotic twins (De Paepe). In the absence of complications*, birth is recommended between 34 and 37+6 weeks gestation (ACOG).


* Monochorionic/Monoamniotic: late division at 8 to 12 days after fertilization; only 2% of monozygotic twins (De Paepe). Due to the risk of cord entanglement and fetal death, in the absence of other complications*, early inpatient admission and management may be offered starting at 24 weeks and birth by cesarean birth is recommended between 32 and 34 weeks gestation (ACOG).


* Conjoined Monochorionic/Monoamniotic: even later zygotic division at 13 to 16 days after fertilization; very rare at 1:100,000 (De Paepe). If expectant management (instead of termination) is desired, birth by cesarean is warranted except in cases of extreme prematurity when neonatal survival is not expected (Malone & D'Alton).



*In about half of all cases of multiples, timing of birth is determined based on complications such as preterm labor, preeclampsia, or poor or discordant fetal growth. (Malone & D'Alton, 2019).


Understanding the type of placentation and chorionicity is critical in providing safe and effective management and care. Following growth closely is an important aspect of prenatal care, especially with monochorionic twins, due to an approximate 15% risk of twin-to-twin transfusion syndrome (Malone & D'Alton, 2019). Twin gestations have a higher risk of preterm birth but carrying twins is not an indication for cesarean birth if the presenting twin is vertex, there are diamniotic sacs, and at least 32 weeks gestation have been completed (ACOG, 2018). Twins carry higher maternal risks including increased incidence of pregnancy-related hypertensive issues (including gestational hypertension and preeclampsia of 10% to 20%) (Malone & D'Alton). Twin pregnancies carry additional risk factors for both mother and babies, and closer care and surveillance are warranted.




College of Obstetricians and Gynecologists. (2018). Multifetal gestations: Twin, triplet, and higher-order multifetal pregnancies (Practice Bulletin No. 169). Obstetrics and Gynecology, 128(4), e131-e146. doi:10.1097/AOG.0000000000001709 [Context Link]


De Paepe M. E. (2019). Multiple gestation: The biology of twinning. In R. Resnik, C. J. Lockwood, T. R. Moore, M. F. Greene, J. A. Copel, & R. M. Silver (Eds.), Creasy and Resnik's maternal-fetal medicine principles and practice (pp. 68-80). Philadelphia, PA: Elsevier. [Context Link]


Malone F. D., D'Alton M. E. (2019). Multiple gestation: Clinical characteristics and management. In R. Resnik, C. J. Lockwood, T. R. Moore, M. F. Greene, J. A. Copel, & R. M. Silver (Eds.), Creasy and Resnik's maternal-fetal medicine principles and practice (pp. 654-675). Philadelphia, PA: Elsevier. [Context Link]