PRACTICAL SONOGRAPHY OF TWIN GESTATIONS: THE THREE ESSENTIALSGwendolyn Bryant-Smith, M. D., Nafisa K . Dajani, M. D.,Lana Glenn, R.D.M.S., Teresita L. Angtuaco, M. D.

University of Arkansas for Medical Sciences, Little Rock, Arkansas

INTRODUCTIONThe incidence of twins has steadily increased in the past 20 years primarily due to fertility treatments. In the United States, twins comprise approximately 1.5 % of all pregnancies yet twin births represent 10% of all perinatal morbidity and mortality. This is significantly higher than what is reported for singleton pregnancies. Preeclampsia, hypertension, placental complications, and postpartum hemorrhage are among the more frequent causes of maternal morbidity. In the fetus, common complications include prematurity, intrauterine growth retardation, and congenital anomalies. In addition, there are other unique complications that occur in monochorionic twin pregnancies which include: twin transfusion, twin embolization, conjoined twinning, and twin reversed arterial perfusion syndrome.

Twin gestations can test the technical and diagnostic skills of the sonographer and sonologist. Therefore, radiologists and obstetricians need a practical approach in monitoring changes that can adversely impact the outcome of these pregnancies. Such an approach should consist of three essential components: determination of chorionicity and amnionicity, determination of appropriateness of growth and the detection of congenital anomalies. Early recognition of abnormalities related to these essentials can lead to management decisions that can improve perinatal outcome.

EMBRYOLOGY I. CHORIONICITY/AMNIONICITYA. Before 10 menstrual weeks, the number

of gestational sacs equals the number of chorions. Beyond ten weeks accurate assessment by counting the gestational sac may not be possible once the intertwin membrane is formed by the opposing amnions.

B. After 10 weeks the following criteria to determine chorionicity may be used:

1. disparate fetal gender – helpful only if one twin is male and the other female

2. number of placentas – two placentas clearly separate from one another imply two chorions

3. appearance of the intertwin membrane - chorionic peak or lambda sign is the extension of placental tissue into the base of the intertwin membrane. The peak is made up of a triangular area of hyperechoic placental tissue that extends between the two sacs. In monochorionic gestations, the intertwin membrane inserts at right angles into the placenta.

C. Amnionicity equals the number of amniotic sacs• amnion may be visible during the 7th to 8th menstrual

week with endovaginal sonography• look for yolk sacs if amniotic membrane is not seen;

although the yolk sac differentiates later than the amnion, it can be demonstrated earlier on ultrasound

• visualization of two yolk sacs imply diamnionicity • failure to see an intertwin membrane in second

trimester or visualization of one yolk sac suggests monoamniot ic gestation

• demonstration of entanglement of the twin umbilical cords is proof of monoamnionicity; confimed by tracing both fetal cords into an entangled mass; Color Doppler may be helpful in demonstrating vascular compromise

SUGGESTED READINGS: 1. Callen, Peter W.;Ultrasonography in obstetrics and gynecology.

W.B. Saunders Company. 4th edition.2000:171-205.

2. Kurtz, Alfred B.; Middleton, William D.; Ultrasound: The Requisites. Mosby. 1996: 341-355.

3. Nyberg,David A.; McGahan,John P.; Pretorius,Delores H.; Pilu,Gianluigi. Diagnostic Imaging of Fetal Anomalies. Lippincott, Williams, and Wilkins. 2003: 778-813.

4. Drugan A, Johnson MP, Krivchenia EL, et al. Genetics and genetic counseling. In: Gall SA,ed. Multiple pregnancy and delivery. St Louis: Mosby, 1996: 85-97.

5. McGahan, John P. Diagnostic Obstretrical Ultrasound. J.B. Lippincott Company. 1994: 434- 448.

6. Beckmann, Charles.; Ling, Frank W.; Herbert, William.; Laube, Douglas W.;Smith, Roger.P.; Barzansky, Barbara M.; Obstretrics and Gynecology. Third Edition. Williams and Wilkins. 1998:

247-242.

DIZYGOTIC TWINS• arise from two separate ova and represent 70% of twin pregnancies• seven days after fertilization, two separate blastocysts implant in the uterine cavity • all dizygotic twins are dichorionic and diamniotic• similarities between dizygotic twins comparable to non-twin siblings

MONOZYGOTIC TWINS

DICHORIONIC DIAMNIOTIC TWINS

• result of all dizygotic twinning• can also result from

monozygotic twinning if the zygote divides within 4 days after fertilization ( before blastocyst formation and chorion differentiation)

• results in two chorions and two amnions• represents approximately 25 %

of all monozygotic gestations

MONOCHORIONIC DIAMNIOTIC TWINS

• division of inner cell mass between 4 and 8 days after fertilization (after chorion differentiates but before amnion differentiates)

• results in one chorion and two amnions

• represents approximately 75 % of monozygotic gestations

MONOCHORIONIC MONOAMNIOTIC TWINS

• division of inner cell mass between 8 and 12 days after fertilization (after amnion differentiates)

• Results in one chorion and one amnion

• Represents approximately 1 %

of monozygotic gestations.

CONJOINED TWINS

• division of the embryonic disk more than 13 days after fertilization

• incomplete division results• always monochorionic

monoamniotic

• develop from one fertilized ovum and make up 30 % of all twins

• timing of the division of the zygote relative to the time of differentiation of the chorion (4 days) and amnion (8 days) determines the chorionicity and amnionicity of twins

Dichorionic Diamnotic Gestation

Transabdominal US Endovaginal US

Dichorionic Diamnotic Gestation with two separate placientas

Dichorionic Gestation with chorionic peak sign

Monochorionic Gestation

Diamniotic Gestation

two yolk sacs

Monochorionic Monoamniotic Gestation with tangled cord

Color Doppler US Gross Specimen

(Reproduced from Suggested Reading 3)

Monochorionic Monoamniotic Gestation with one yolk sac

two amniotic sacs

I I . GROWTHIt is important to determine if twin growth is concordant or discordant. From measurements of fetal head, abdomen and femur, the fetal weights are estimated and compared. The difference between two fetal weights is divided by the weight of the larger twin to determine percent discordance. Discordant growth is defined by a difference in birthweight of 20% or more. Discordant growth leads to significant increase in morbidity and mortality as compared to twin gestations with concordant growth. Discordant growth may be due to chromosomal abnormalities, in utero infection or placental abnormalities. In monozygotic gestations vascular shunting between twins through the placenta may be the source of major discordance in size resulting in Twin-Twin transfusion syndrome. Because of these complications, twins are monitored every 3-4 weeks to detect the earliest signs of compromise.

CONCORDANT GROWTH

OMPHALOCELE

DOWN SYNDROME

B. ABNORMALITIES SPECIFIC TO MONOCHORIONIC TWINS

In addition to structural anomalies, there are unique abnormalities unique to monozygotic twins. They include twin transfusion syndrome, twin embolization, conjoined twinning and twin-reversed arterial perfusion sequence.

TWIN-TWIN TRANSFUSION SYNDROME • complication of 15-30% of monochorionic twin gestations• commonly occurs in the late second or third trimester• result of unbalanced flow of blood across the placental vascular

communications from one donor fetus to a recipient fetus• one twin usually has marked polyhydramnios and the other twin

usually has oligohydramnios• usually a birthweight discrepancy of greater than 20 %• larger twin also frequently plethoric and hydropic• smaller fetus is usually anemic and may appear stuck or fixed to the

uterine wall (stuck twin syndrome)• stuck twin is not pathognomonic for twin-twin transfusion; can

also be seen in IUGR of smaller twin without associated placental anastomosis

TWIN EMBOLIZATION • tissue necrosis in a living twin secondary to in utero co-twin demise• observed in cases where co-twin demise occurs later in pregnancy

usually after 20 weeks; co-twin may develop an infarct or necrosis of highly vascular organs, i.e. the brain, liver, and kidneys

• early loss occurring in the 1st trimester usually has little effect on the surviving fetus

I I I . FETAL ABNORMALITIESCONJOINED TWINS• rare anomaly of monochorionic, monoamniotic

gestations which occurs when the embryonic disk incompletely divides more than 13 days after fertilization

• classified according to the location of fusion

TWIN- REVERSED ARTERIAL PERFUSION SEQUENCE (ACARDIAC TWIN)• rare anomaly which occurs in 1% of monozygotic twin pregnancies• anomaly is the most extreme manifestation of Twin-Twin transfusion• characterized by a severely

malformed co-twin with either an absent heart or a nonfunctional cardiac structure

• acardiac fetus is considered a parasite because of its need for pumped blood from the normal twin

• associated with arterial to arterial and venous to venous placental anastomosis causing an imbalance of the interfetal circulation leading to reversed blood flow in the umbilical artery of the recipient twin which causes secondary atrophy of the heart

• diffuse edema and cystic hygroma are usually present in the acardiac twin

• Doppler evaluation show reversed flow in the umbilical cord of the acardiac twin

CONCLUSIONTwin gestations are high risk pregnancies for a variety of reasons. In order to perform a comprehensive assessment, knowledge of embryology and classification of twins is a primary requirement. Familiarity with sonographic findings of associated abnormalities is crucial. Each twin gestation should be assessed according to three essential components in a step by step algorithm designed to take into consideration every aspect of the twinning phenomenon. It is imperative that the chorionicity and amnionicity be determined as a first step. This is followed by a decision about concordance or discordance of growth that can lead to alteration in management. Because of the higher incidence of congenital anomalies, a much more rigorous search for abnormalities should be undertaken. Increased awareness of the unique circumstances surrounding twins can help decrease their associated high mortality and morbidity.

CRANIOPAGUS

PYGOPAGUS

OMPHALOPAGUS

DISCORDANT GROWTH

Endovaginal US

two yolk sacs

Monochorionic Monoamniotic Gestation with one yolk sac

Twins showing symmetric growth

Twins with asymmetric growth

A. ABNORMALITIES NOT SPECIFIC TO TWINS

These types of abnormalities found in twins are not any different from those seen in singletons. However, the incidence of congenital anomalies has been found to be 2.5 times more prevalent in monozygotic twins than in dizygotic twins or singleton gestations. More specifically, chromosomal abnormalities have been found to be increased two-fold in dizygotic twins compared with singleton gestations.

THANATOPHORIC DWARFS

Clover-leaf skull and hydrocephalus in twin A

Clover-leaf skull and hydrocephalus in twin B

Normal abdomen twin A Omphalocele twin B

Normal twin A Twin B with nuchal lucency

hydropic recipient twin andgrowth- retarded donor twin donor twin( ) in a “stuck” position

with intertwin membrane ( )demonstrated

polyhydramnios and ascitesin hydropic twin

prenatal images showing cerebral infarction ( ) in living twin with co-twin demise

sagittal and axial MRI of the neonate showing massive

infarction

ISCHIOPAGUS

twins with ischiopagus at 12 weeks

transverse ultrasound image of twins with omphalopagus

at 30 weeks

THORACOPAGUS

ultrasound and gross images of thoracopagus with parasitic twin

Ulrtasound and MRI images of dicephalus twins

coronal ultrasound image of the head and gross image of diprososus twins with incomplete division of the head with

two faces and shared cheek

diagram of direction of vascular flow between twins

acardiac amorphous twin ( )

acardiac hydropic (B) twin and normal twin (A)