Monoamniotic Twin Pregnancies

A twin pregnancy is defined as “monoamniotic” in the presence of a single placenta with one amnion and one chorion.

O30.0

Monoamniotic twin pregnancies are rare, as they represent the 0.01% of all spontaneous pregnancies, with a reported incidence of about 1 per 10,000 pregnancies. (1)  The incidence of these pregnancies is higher in case of in vitro fertilization (IVF), particularly when including embryo manipulation or assisted hatching. (2)
Monoamniotic twin pregnancies also account for 1% of all twin pregnancies and 5% of monochorionic gestations. (1) 
Interestingly, in monoamniotic pregnancies the proportion of female twin pairs is significantly higher (about two-thirds), compared with male twin pairs, likely due to a delay of the embryonic splitting that is preceded by the process of X-chromosome inactivation in female embryos. (3-4)

Monoamniotic pregnancies usually result from a single fertilized egg; in this case, the embryo splits between 8 and 13 days after fertilization. Conjoined twin pregnancies are a subset of monoamniotic pregnancies that result from a later embryonic splitting (after 13 days) and are even more rare (1.5 per 100,000 pregnancies). (2,5)
Moreover, monoamniotic pregnancy may be the consequence of intermembrane rupture in diamniotic pregnancies, which can be spontaneous or related to invasive procedures (i.e., amniocentesis or fetal surgery). This condition is usually defined as spontaneous or iatrogenic “septostomy”. (2)

All monoamniotic pregnancies are considered high-risk pregnancies. 

Complications of monoamniotic twin pregnancies can be divided into either those those peculiar to both monochorionicity and monoamnionicity and those occurring in all twin pregnancies.
When focusing on typical complications of monochorionic pregnancies, similar to diamniotic pregnancies monoamniotic pregnancies are also at risk of developing twin-to-twin transfusion syndrome (TTTS); twin reversed arterial perfusion (TRAP) sequence, also known as acardiac twinning; and selective fetal growth restriction (sFGR). Compared to diamniotic monochorionic pregnancies, TTTS occurs less frequently in monoamniotic pregnancies, with an incidence ranging from 2 to 6%. (3,6) The reason for such a lower frequency of TTTS has been postulated to be associated with the almost ubiquitous presence of compensating arterio-arterial anastomoses in monoamniotic placentas that might play a role in preventing TTTS. (6) Conversely, for this same reason the incidence of TRAP sequence has been reported to be higher in monoamniotic pregnancies.

Monoamniotic pregnancies also face other complications that are specific of monoamnionicity, and in particular cord entanglement and conjoined twins. Cord entanglement occurs virtually in all monoamniotic pregnancies, and the main concern is that intermittent occlusion of umbilical blood vessels may cause neurologic morbidity or even fetal death in case of severe prolonged occlusion. However, the overall survival rate in case of cord entanglement was around 89%, with no significant difference in terms of mortality in monoamniotic pregnancies with or without entanglement and no improvement of perinatal outcomes even after sonographic visualization of cord entanglement. (7-8) Finally, conjoined twins are a rare type of monoamniotic pregnancies in which one monoamniotic twin has a part of the body fused with the other, and according to the anatomical region that is fused can be differed into cephalopagus, thoracopagus, omphalopagus, ischiopagus, parapagus, craniopagus, rachipagus, and pygopagus. More than half of conjoined twin pregnancies results in fetal loss. (2)

In addition, monoamniotic twins face the complications that exist in all twin pregnancies, such as spontaneous preterm birth, gestational diabetes, hypertensive disorders of the pregnancy, preeclampsia, severe cardiovascular disorders, such as stroke and deep venous thrombosis, and cholestasis. (2)

Monoamniotic pregnancies carry a higher risk of developing congenital anomalies compared to either singleton and dichorionic and monochorionic diamniotic pregnancies, with an estimated prevalence ranging from 10 to 25%. The most common anomalies include congenital cardiac defects (30% of all cases). Structural anomalies might be discordant between the two fetuses, often with only one fetus being affected. Among these cases, chromosomal anomalies are relatively rare (4%); the underlying mechanism might be more likely related to hemodynamic disequilibrium within the monochorionic placental anastomoses. (2,5)

The diagnosis of chorionicity and amnionicity should be performed in the first trimester scan, between 11 and 13+6 weeks of gestation. In this scenario, there are some ultrasound characteristics that may suggest monoamnionicity such as (9-12):
-    Lack of an intertwin membrane
-    One yolk sac with two fetal poles (although the number of yolk sacs is not a reliable predictor of amnionicity, as the presence of two sacs has been reported also in monoamniotic pregnancies)
-    Close placental insertion of the umbilical cords and early cord entanglement 
Signs of monochorionicity (Single placenta, same sex of fetuses) 

No specific recommendation on how to manage monoamniotic twins is provided by national and international guidelines. 

The ultrasound surveillance in uncomplicated monochorionic pregnancies include (12):
-    First trimester scan for the definition of chorionicity and amnionicity and to correctly date the pregnancy (11-13+6 weeks)
-    First trimester screening (11-13+6 weeks) for aneuploidies, including the measurement of nuchal translucency, the Doppler of the ductus venosus, the tricuspid regurgitation; first trimester screening for preeclampsia (measurement of uterine artery Doppler)
-    Monitoring fetal growth, amniotic fluid quantity and umbilical artery Doppler for the screening of TTTS, every 2 weeks starting from 16 weeks of gestation (from 20 weeks onward, this screening scan also includes the measurement of the peak systolic velocity of the middle cerebral artery (PSV-MCA) for the screening of twin anemia polycythemia sequence (TAPS)
-    Detailed anatomy scan at 18-22 weeks of gestation; screening for preterm birth (cervical length)
-    Fetal echocardiography at 18-22 weeks of gestation 
-    Continue every-2-weeks surveillance of fetal growth, amniotic fluid quantity, umbilical and middle cerebral artery Doppler.

In third trimester, assessment of growth discordance can be useful to strengthen surveillance, as the risk of intrauterine death has been reported to be significantly and increasingly higher in pregnancies with birthweight discordance from ≥ 10% to ≥ 30%, despite with low predictive accuracy for mortality. (13)

Further studies are needed in order to identify a tailored approach to the clinical surveillance of monochorionic monoamniotic pregnancies. 

The risk of aneuploidy in monoamniotic pregnancies has been reported to be similar or lower compared to that of a singleton gestation. (14) Usually both twins are affected with rare exceptions due to postzygotic and epigenetic mechanisms. The prenatal screening for aneuploidies in monoamniotic pregnancies can be performed either by using standard screening, including NT and biochemical markers, or cfDNA, which has been reported to be superior to standard assessment in monochorionic pregnancies. (14) If fetal anomaly is present in one of both the twins, an invasive procedure should be considered. 

The overall survival rate of monoamniotic twins has increased over the last few decades. (13) However, among twin pregnancies, monoamniotic are those at the highest risk of mortality both earlier and later in gestation, with risk of fetal death that is 9 times higher in monoamniotic compared with dichorionic twin pregnancies even after 24 weeks of gestation. (2)

The most recent evidence on the outcome of non-anomalous monoamniotic twins reaching 24 weeks of gestation showed that overall fetal death occurred in 5.8% of fetuses, and the rate of single and double intrauterine deaths was 2.5% and 3.8%, respectively. Fetal demise occurred in 4.3% of twins at 24-30 weeks, in 1.0% at 31-32 weeks and in 2.2% at 33-34 weeks of gestation. (15)

Alongside concerns regarding mortality, monoamniotic pregnancies have also been reported to be at higher risk of neonatal morbidity. The rate of composite morbidity has been showed to be about 46% and is mainly related to respiratory morbidity and prematurity, with two third of newborns admitted to NICU, regardless of gestational age at delivery. Also, neonatal morbidity appears to gradually decrease while increasing gestational age at delivery, with a significant decrease at 33-34 weeks. (16)

Clinical surveillance of monoamniotic pregnancies may be performed either with inpatient or outpatient care.

Recent studies comparing the setting of the management showed that in uncomplicated monoamniotic twins, inpatient surveillance was associated with similar fetal mortality as outpatient management, thus leading to the hypothesis that the quality of surveillance is more important than the setting. (13,17) However, in most clinical settings monoamniotic twins are currently managed as inpatients.

The prospective risk of a non-respiratory neonatal complication in non-anomalous neonates has been reported to decrease suddenly after 32 weeks of gestation and at 32+4/7 the prospective risk of intrauterine fetal death (3.1%) exceeds that of non-respiratory neonatal complication. (18)

As such, international guidelines recommend elective preterm delivery at 32-34 weeks of gestation. Caesarean delivery is often preferred by most clinicians and recommended by most international guidelines to avoid an umbilical cord complication of the non-presenting twin at the time of the initial twin’s delivery. (19-21)

1.    Glinianaia SV, Rankin J, Khalil A, et al. Prevalence, antenatal management and perinatal outcome of monochorionic monoamniotic twin pregnancy: a collaborative multicenter study in England, 2000-2013. Ultrasound Obstet Gynecol 2019; 53:184–92
2.    Van Mieghem T, Abbasi N, Shinar S, et al. Monochorionic monoamniotic twin pregnancies. Am J Obstet Gynecol MFM. 2022; 4(2S):100520
3.    Hack KE, Derks JB, Schaap AH, Lopriore E, Elias SG, Arabin B, Eggink AJ, Sollie KM, Mol BW, Duvekot HJ, Willekes C, Go AT, Koopman-Esseboom C, Vandenbussche FP, Visser GH. Perinatal outcome of monoamniotic twin pregnancies. Obstet Gynecol. 2009; 113(2 Pt 1):353-60. 
4.    Monteiro J, Derom C, Vlietinck R, Kohn N, Lesser M, Gregersen PK. Commitment to X inactivation precedes the twinning event in monochorionic MZ twins. Am J Hum Genet 1998; 63:339–46
5.    Khairudin D, Khalil A. Monochorionic monoamniotic twin pregnancies. Best Pract Res Clin Obstet Gynaecol. 2022; 84:96-103. 
6.    Hack KE, van Gemert MJ, Lopriore E, Schaap AH, Eggink AJ, Elias SG, van den Wijngaard JP, Vandenbussche FP, Derks JB, Visser GH, Nikkels PG. Placental characteristics of monoamniotic twin pregnancies in relation to perinatal outcome. Placenta. 2009; 30:62-5. 
7.    Dias T, Mahsud-Dornan S, Bhide A, Papageorghiou AT, Thilaganathan B. Cord entanglement and perinatal outcome in monoamniotic twin pregnancies. Ultrasound Obstet Gynecol. 2010; 35:201-4.
8.    Rossi AC, Prefumo F. Impact of cord entanglement on perinatal outcome of monoamniotic twins: a systematic review of the literature. Ultrasound Obstet Gynecol. 2013; 41:131-5. 
9.    Townsend RR, Filly RA. Sonography of nonconjoined monoamniotic twin pregnancies. J Ultrasound Med. 1988; 7:665-670. 
10.    Babinszki A, Mukherjee T, Kerenyi T, Berkowitz RL, Copperman AB. Diagnosing amnionicity at 6 weeks of pregnancy with transvaginal three-dimensional ultrasonography: case report. Fertil Steril. 1999; 71:1161-1164. 
11.    Bishop DK. Yolk-sac number in monoamniotic twins. Obstet Gynecol. 2010; 116 Suppl 2:504-507. 
12.    Khalil A, Rodgers M, Baschat A, et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy [published correction appears in Ultrasound Obstet Gynecol. 2018 Jul;52(1):140]. Ultrasound Obstet Gynecol. 2016; 47:247-263. 
13.    Saccone G, Khalil A, Thilaganathan B, Glinianaia SV, Berghella V, D'Antonio F; MONOMONO, NorSTAMP and STORK research collaboratives. Weight discordance and perinatal mortality in monoamniotic twin pregnancy: analysis of MONOMONO, NorSTAMP and STORK multiple-pregnancy cohorts. Ultrasound Obstet Gynecol. 2020; 55:332-338. 
14.    D'Antonio F, Khalil A. Screening and diagnosis of chromosomal abnormalities in twin pregnancy. Best Pract Res Clin Obstet Gynaecol. 2022; 84:229-239. 
15.    D'Antonio F, Odibo A, Berghella V, et al. Perinatal mortality, timing of delivery and prenatal management of monoamniotic twin pregnancy: systematic review and meta-analysis. Ultrasound Obstet Gynecol. 2019; 53:166-174. 
16.    Buca D, Di Mascio D, Khalil A, et al. Neonatal Morbidity of Monoamniotic Twin Pregnancies: A Systematic Review and Meta-analysis. Am J Perinatol. 2022; 39:243-251
17.    MONOMONO Working Group. Inpatient vs outpatient management and timing of delivery of uncomplicated monochorionic monoamniotic twin pregnancy: the MONOMONO study. Ultrasound Obstet Gynecol. 2019; 53:175-183. 
18.    Van Mieghem T, De Heus R, Lewi L, Klaritsch P, Kollmann M, Baud D, Vial Y, Shah PS, Ranzini AC, Mason L, Raio L, Lachat R, Barrett J, Khorsand V, Windrim R, Ryan G. Prenatal management of monoamniotic twin pregnancies. Obstet Gynecol. 2014; 124:498-506
19.    Management of Monochorionic Twin Pregnancy: Green-top Guideline No. 51. BJOG. 2017; 124(1):e1-e45. 
20.    American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics, Society for Maternal-Fetal Medicine. Multifetal Gestations: Twin, Triplet, and Higher-Order Multifetal Pregnancies: ACOG Practice Bulletin, Number 231. Obstet Gynecol. 2021; 137(6):e145-e162.  
21.    Twin and Triplet Pregnancy. London: National Institute for Health and Care Excellence (NICE); September 4, 2019.

Please note that the maximum upload size is 5MB, and larger images and video clips can be sent to [email protected].