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Perinatal outcome of monochorionic in comparison to dichorionic twin pregnancies.


There are two types of twins: dizygotic and monozygotic twins. Approximately two thirds of twins are dizygotic and they result from fertilization of two or more eggs by different spermatozoa, with an incidence of 7 to 11 per 1000 births. The second type of twins is monozygotic, which results from the splitting of the zygote at various stages of development with an incidence of 3 to 4 per 1000 births. (1) Different types of twins arise from monozygotic twins depending on the time of cleavage after fertilization as described in Table 1. (2)

Advanced maternal age and use of reproductive techniques have increased the incidence of multiple pregnancies over the last decades. In the United States, the incidence of these pregnancies has increased by 59% between 1980 and 1999. Approximately 130,000 infants born in the United States are twins out of 4 million every year. (3) Although monochorionic twins account for 3% of the entire group, they present 16% of the total perinatal mortality. Monochorionic twins have the highest risk of congenital malformations, perinatal mortality and morbidity. (4) The incidence of monochorionic twins is very low, the outcomes for this kind of twins and the best gestational age for delivery have not been well studied. More studies are needed, and thus the aim of this study was to look into the neonatal outcomes of monochorionic twin pregnancies including the gestational age of delivery and to compare it with dichorionic twins. Although similar studies were reported in the literature, no study has yet been conducted in Oman. Different populations and different obstetric practices may modify rates of pregnancies with multiple gestations, as well as morbidity and mortality seen in this group.


A retrospective cohort study of twin pregnancies was conducted at Sultan Qaboos University Hospital (SQUH) between January 2006 and December 2011. This is a tertiary care hospital with around 4,000 deliveries per year and a cesarean section rate of around 18%. The data of women with twin pregnancies and their infants were obtained from the medical records. This study was approved by Sultan Qaboos University-based ethical review committee. Two sets of twins were excluded because they were lost for follow-up.

Gestational age (GA) was determined by the last menstrual period or by ultrasound before 20 weeks of gestation. Chorionicity was determined by ultrasound examination in the antenatal period and confirmed by histological examination of the placenta in the postnatal period. Chorionicity was classified as: dichorionic diamniotic (DCDA), monochorionic diamniotic (MCDA) and monochorionic monoamniotic (MCMA). Twin to twin transfusion syndrome (TTTS) was diagnosed by the recognition of a single placenta, twin's weight discordance, same gender, intrauterine growth restriction in the donor, normal or large size of the recipient, bladder sizes and abnormal Doppler's at later stages. Growth discordant was diagnosed if the inter-pair difference in estimated fetal weights (EFW) was [greater than or equal to] 20% (EFW large- EFW small/EFW large x 100). (5)

Preterm birth was defined as delivery before 37 weeks of gestation. Newborns weighing less than 2500 g was classified as low birth weight. Intrauterine fetal death (IUFD) was defined as death of the fetus inutero after 20 weeks gestation. Fetal hydrops was diagnosed with the presence of fluid in two or more fetal cavities. Respiratory distress syndrome (RDS) was diagnosed based on clinical and radiographic parameters. Congenital malformations were diagnosed by ultrasound examination during the antenatal period or by clinical examination of the neonate. The following data were recorded for each case: maternal demographics including age, gravidity, parity, past obstetrical history, previous surgeries, past medical history, last menstrual period, obstetrical complications during current pregnancy and neonatal outcome including: birth weight (BW), neonatal intensive care unit (NICU) admission, and complications of prematurity (Intraventricular hemorrhage, retinopathy of prematurity, respiratory distress syndrome, necrotizing enterocolitis, sepsis, death).

Continuous variables are reported as means with standard deviation (SD) and ranges. Catergorical variables are reported as frequencies and proportions. Chi-square test was used to compare between categorical variables and student t-test was used to compare between means. And p values of <0.05 is used for statistical significance.


There were 51 sets of twin pregnancies in the study. Forty-six (90%) of the mothers were Omanis. The mean maternal age was 27.1 years (SD 4.1) and 15 (29%) of the mothers were primigravida. Pre-eclampsia was noted in 9 (18%) mothers. The most frequent route of delivery was cesarean section (94%) followed by normal vaginal delivery (6%), (Table 2). With respect to chorionicity, 36 (71%) pregnancies were DCDA, 14 (27%) were MCDA, and one (2%) was MCMA.

The Antepartum complications noted in the 15 monochronoic twins included: discordant fetal growth in 2 (13%) cases, low birth weight in 11 (73%) babies, pre-eclampsia in 3.

(20%) mothers, and TTTS in 4 (26.7%) sets. Fetal respiratory distress affected 8 (53%) pregnancies. Antepartum steroid for fetal lung maturity was given in 13 mothers (87%). Six (40%) twin sets were delivered before 30 weeks, 4 (27%) sets at 31-32 weeks, 2 sets (13%) at 35-36 weeks, 2 sets (13%) at 36-37 weeks and 1 set (7%) at 37-38 weeks. Fifteen mothers delivered 16 live infants (53%), 9 were stillbirths (30%) and 5 babies (17%) died after birth. Most neonatal mortality was the result of neonatal sepsis and pulmonary hypoplasia. (Tables 3 and 4)

The outcomes for monochorionic twins were as follows:

1. Prematurity--which was seen in 13/15 (87%) prenancies delivered preterm

2. Twin-to-twin transfusion syndrome

There were four cases (27%) of TTTS. Two of the complicated cases are detailed below.

Case 1. This MCDA twin pregnancy was diagnosed with TTTS at 23 weeks gestation. The diagnosis was based on the presence of polyhydramnios/oligohydramnios sequence, fetal growth discordance and abnormal Doppler studies in the intrauterine growth-restricted twin. The larger twin progressed to fetal hydrops and the smaller infant was complicated by IUFD.

Case 2: This MCDA twin pregnancy was diagnosed at 22 weeks with TTTS. The larger twin developed fetal hydrops at 25 weeks gestation and was stillborn and the smaller twin was born alive and died after 4 hours. The twin had mildly dilated lateral ventricles.

There were two sets of MCDA twins which resulted in IUFD at 21 and 28 weeks gestation respectively, but the cause was unknown.


In this study, there was only one set of MCMA twins. The mother was a primigravida who conceived spontaneously and had close follow-up during the pregnancy. She was admitted from 28 weeks gestation for fetal monitoring. No maternal or fetal complications were identified and after extensive counseling, the decision was made for cesarean delivery at 34 weeks. Unfortunately, sudden IUFD of one twin occurred at 32 weeks and an emergency cesarean section was performed. The death was attributed to cord entanglement. The surviving twin did well and MRI at first week of life and at 6 months did not show any hypoxic insult.

For the dichorioinic twins (DC), the mortality rate was significantly lower in DC (17%) when compared to MC (47%) twins (p = 0.017). The mean gestational age at delivery was 36.3 weeks ([+ or -] 3.7) for DC twins and 34.3 weeks ([+ or -] 3.6) for MC twins (p = 0.035). The mean birth weight of DC twins was 1799 g (SD: 701) and 2055 g ([+ or -] 715) for MC twins (p = 0.18). The frequency of cesarean deliveries among DC twins was 14 (93%) compared with 31 (86%) for MC twins (p = 0.22). The comparison between dichorioinic and monochorionic twins is illustrated in Table 5.


Our study has shown a 47% mortality rate of monochorionic pregnancies mainly attributed to TTTS. This high rate could be explained by late presentation to our hospital and end stages of TTTS that had a poor prognosis. Monchrorionic pregnancies have high perinatal mortality of about 68%. (6,7) This high rate could be explained by prematurity, TTTS, umbilical cord entanglements and congenital malformations. (6,7) Before 1990, antenatal chorionicity determination was difficult; therefore, antenatal diagnosis of monchrorionic pregnancies was made in less than half of the patients and the majority were delivered vaginally. (8) Over the last decades significant reduction in perinatal mortality has been reported, 40 to 60% in 1961 to 1962, reduced to 28% in 1961 to 1982, to 20% in 1997 to 1999 and 12% in 2001 to 2004. (8,9,10) These changes in perinatal mortality can be explained by advanced fetal and neonatal management including antenatal diagnosis of twin type, close fetal surveillance, recognition of complications and early antenatal interventions.

The incidence of premature birth of twin pregnancies is higher than singletons and the monchrorionic twins are even at much greater risk. In our study, the mean gestational age of monochorionic twins at delivery was 34.3 weeks with 87% of deliveries occurring before 37 weeks. The mean age at delivery of twin pregnancies reported in previous studies was 35.6 weeks; this difference can be explained by the high frequency of pregnancies with complications referred to our Hospital. According to chorionicity, the mean gestational age at delivery was lower in monochorionic pregnancies compared to dichorionic pregnancies. Our study showed the mean gestational age of 36.3 week for dichorionic pregnancies and of 34.3 week for monochorionic pregnancies. In comparison with another study conducted at the Sao Paulo University School of Medicine, the mean gestational age at delivery was 36.7 weeks for dichorionic pregnancies and 35.6 weeks for monochorionic pregnancies. (11)

Low birth weight (LBW) was mostly seen in twin newborns than among singleton newborns. Birth weight below 2500 g is 8.3 times more common in twins in comparison to singleton and birth weights less than 1500 g is 10 times more common in twins. (12) Classification regarding to chorionicity in our study showed a mean BW of 2055 g for dichorionic pregnancies and 1799 g for monochorionic pregnancies. Previous studies showed mean BW of 2165 g for dichorionic pregnancies and of 1827 g for monochorionic pregnancies almost similar to our study results. (11)

Twin-to-twin transfusion syndrome (TTTS) is more common in MCDA than MCMA pregnancies because MCDA placentas have less superficial arterio-arterial and more deep arteriovenous anastomoses than MCMA placentas. (13,14) We report of TTTS in 4 of the 15 monochorionic pregnancies (27%), whereas TTTS was reported in previous studies as 8%. (15) This difference could be explained by the small sample size of monochorionic twin gestations in our study and due to the large number of high risk pregnancies referred to our center.

The optimal time for delivery of monochorionic twins to reduce the risks from cord entanglement, growth discrepancies and intrauterine fetal death is still controversial. However, most literatures, reported that the optimal time for delivery of MCMA twins is no later than 32 to 34 weeks of gestation. (7,10) Premature infants, especially those born before 32 weeks of gestation, have a high risks of perinatal depression, respiratory distress, patent ductus arteriosus, early-and late-onset sepsis, intracranial hemorrhage, necrotizing enterocolitis,, hospitalization and neurological complications. (16) Our study had only one set of MCMA (2%) who were planned for delivery at 34 weeks but delivered at 32 weeks gestation due to sudden IUFD of one twin. Fortunately, the other twin was born alive and was discharged from hospital with no evidence of hypoxic insult.

The incedence of maternal and fetal complications increase significantly in twin pregnancies. As a result of the increase of twin pregnancies over the last decades, obstetricians must be the aware of the adverse neonatal outcomes in this type of pregnancy. It is very important to determine the chorionicity at the beginning of pregnancy as each type carries different prognosis and expected morbidity. This will also help towards counseling the parents. Chorionicity can be determined in the first trimester by ultrasound examination. Moreover, regular ultrasound follow-up can help in predicting of pregnancies that are at a higher risk of fetal growth restriction and discordant growth. (17)

The current study limitations include small sample size, retrospective study at one tertiary center and the low incidence of MCMA twins. Further studies involving more hospitals are needed to verify the current findings.


Monochorionic twins remain a group that carry high perinatal morbidity and mortality rates because they are more likely to involve twin-to-twin transfusion syndrome (TTTS), prematurity, fetal growth restriction and IUFD. However, improved fetal and neonatal management may result in better outcomes.

DOI 10.5001/omj.2013.49

Received: 11 Jan 2013/Accepted: 27 Mar 2013


There was no conflict of interest between the authors in conducting this study and no funding was received for this work.


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Nihal Al-Riyami ([mail])

Department of Obstetrics and Gynecology, Sultan Qaboos University Hospital, Al

Khoud, Sultanate of Oman.


Asmaa Al-Rusheidi

2nd Year Medical Student, Sultan Qaboos University, Al Khoud, Sultanate of Oman.

Murtadha Al-Khabori

Department of Hematology, Sultan Qaboos University Hospital, Al Khoud, Sultanate

of Oman.
Table 1: Different types of twins from
monozygotic twins.

Time of cleavage      Nature of membranes
after fertilization

0-3 days              Dichorionic Diamniotic (DCDA)
4-8 days              Monochorionic Diamniotic (MCDA)
9-12 days             Monochorionic Monoamniotic (MCMA)
13 days or more       Conjoint twins

Table 2: Maternal demographics.

Demographics                               Value

Pregnant patients no.                       51
Maternal age (years) ([+ or -] SD)   27.1 [+ or -] 4.1
Primigravida no. (%)                      15 (29)
Preeclampsia no. (%)                      9 (18)
Cesarean delivery no. (%)                 48 (94)

Table 3: Monochorionic pregnancy complications (15).

Complications                              No.(%)

Discordant fetal growth                    2 (13)
Low birth weight                           11 (73)
Preeclampsia                               3 (20)
Twin-to-twin transfusion                   2 (13)
Fetal respiratory distress                 8 (53)
Antepartum steroids                        13 (87)
[less than or equal to] 30 weeks           6 (40)
  gestational age at delivery
31-32 weeks gestational age at delivery    4 (27)
33-34 weeks gestational age at delivery     0(0)
35-36 weeks gestational age at delivery    2 (13)
36-37 weeks gestational age at delivery    2 (13)
37-38 weeks gestational age at delivery     1 (7)

Table 4: Perinatal outcome in monochoroinic twin pregnancies

N          Pregnancy   Complication                 GA

1          Twin A      TTTS (donor)              23 weeks
           Twin B      TTTS (recipient)
2          Twin A      TTTS (donor)              25 week
           Twin B      TTTS (recipient)
3          Twin A      Cystic structure 47.1 x   21 weeks
                         24.1mm in the
           Twin B      post wall of uterus
4          Twin A      None                      32 weeks
           Twin B      None
5          Twin A      None                      32 weeks
           Twin B      None
6 (MCMA)   Twin A      None                      32 weeks
           Twin B      None
7          Twin A      TTTS (donor)              35 weeks
           Twin B      TTTS (recipient) with
8          Twin A      None                      35 weeks
           Twin B      None
9          Twin A      None                      32 weeks
           Twin B      None
10         Twin A      TTTS                      22 weeks
           Twin B
11         Twin A      None                      37 weeks
           Twin B      None
12         Twin A      None                      38 Weeks
           Twin B      None
13         Twin A      Missed abortion for       16 weeks
                         both twin confirmed,
           Twin B        with evidence of
                         minimal abdominal
                         ascites in twin B
14         Twin A      None                      36 weeks
           Twin B      None
15         Twin A      Cord around neck,         28 weeks
           Twin B      Stillbirth

N          Pregnancy   BW (g)           Outcome

1          Twin A      520 gm            (IUFD)
           Twin B      510 gm            (IUFD)
2          Twin A      750 gm            (IUFD)
           Twin B      530 gm       Died after (4h)
                                     from delivery
3          Twin A        --              (IUFD)
           Twin B        --              (IUFD)
4          Twin A      2550 gm          Survived
           Twin B      2830 gm          Survived
5          Twin A      1850 gm          Survived
           Twin B      2100 gm          Survived
6 (MCMA)   Twin A      3475 gm            IUFD
           Twin B                       Survived
7          Twin A      1950 gm          Survived
           Twin B        --               IUFD
8          Twin A      1600 gm          Survived
           Twin B      2000 gm          Survived
9          Twin A      2330 gm          Survived
           Twin B      2160 gm          Survived
10         Twin A      450 gm          Stillbirth
           Twin B      360 gm          Stillbirth
11         Twin A      2205 gm          Survived
           Twin B      2500 gm          Survived
12         Twin A      1835 gm          Survived
           Twin B      2050 gm          Survived
13         Twin A        --         Missed abortion
                                     for both twin
           Twin B        --
14         Twin A      2200 gm          Survived
           Twin B      1997 gm          Survived
15         Twin A      420 gm    Died after few minutes
           Twin B        --            Delivered
                                  macerated stillbirth

BW: birth weight, TTTS: twin to twin transfusion syndrome,
IUFD: intrauterine fetal death, MCMA: monochorionic

Table 5: Perinatal outcome in 51 fetuses of twin pregnancies
according to chorionicity.

Variable            Monochorionic (MC)    Dichorionic (DC)   p value

                      MCMA      MCDA        DCDA No. (%)
                    No. (%)   No. (%)

Fetuses/Neonates         15 (29)              36 (71)
Mortality                14 (47)              6 (17)          0.017
Gestational age of   4.3 [+ or -] 3.6     36.3 [+ or -] 3.7   0.035
  delivery (weeks)
Birth weight (g)    1799 g [+ or -] 701  2055 g [+ or -] 715  0.18
Cesarean delivery        14 (93)              31(86)          0.22
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Author:Al Riyami, Nihal; Al-Rusheidi, Asmaa; Al-Khabori, Murtadha
Publication:Oman Medical Journal
Article Type:Report
Geographic Code:1USA
Date:May 1, 2013
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