Management of Monochorionic – Diamniotic Twins

Contributor: Bill Mabie, MD
Last Update: 5/30/2014

Monochorionic/diamniotic (Mo/Di) twins are at risk for 3 major complications: 1) twin-to-twin transfusion syndrome (TTTS); 2) selective intrauterine growth restriction (sIUGR); and 3) twin anemia-polycythemia sequence (TAPS).  All monochorionic-diamniotic twins should be referred to MFM for consultation and co-management. 

Diagnosis of TTTS is based on oligohydramnios in one gestational sac and polyhydramnios in the other (Oli/Poly).  The maximum amniotic fluid vertical pocket (MVP) is < 2/cm in one sac and MVP is > 8 cm in the other.

Quintero stages of TTTS are as follows:

Stage 1: MVP < 2 cm / MVP > 8 cm, Dopplers in both twins normal.

Stage 2: Oli/Poly and bladder of donor not visualized for one hour. Dopplers in both twins are normal.

Stage 3: Oli/Poly, non-visualized bladder, and abnormal Dopplers (AEDF, REDF, reversed a-wave in ductus venosus, pulsatile flow in umbilical vein).

Stage 4: Fetal hydrops of one or both fetuses. 

Stage 5: Death of one or both fetuses.

Diagnosis of sIUGR:
EFW < 10th percentile in one twin for that gestational age

Diagnosis of TAPS:
Peak systolic velocity (PSV) in MCA Doppler  > 1.5 MOM in donor and PSV in MCA Doppler < 1.0 MOM in recipient. 

These 3 complications tend to occur at different times in pregnancy, but they are interchangeable.  For example, spontaneous TAPS may evolve into TTTS; laser treatment for TTTS may cause TAPS; TTTS and sIUGR may occur in the same patient. 

Brief Summary of the Pathophysiology of these Conditions
TTTS complicates about 8-10 % of Mo/Di twins.  It is usually detected between 16 – 26 weeks, but it can occur from 14 weeks to term. It is due to a large A-V shunt in the placenta that causes a volume shift between the fetuses. There are artery to artery, vein to vein, vein to artery, and artery to vein anastomoses in virtually all monochorionic twin placentas.  The abnormal shunt occurs at the placental cotyledon level where an artery from one fetus goes into a cotyledon and a vein comes out of that cotyledon carrying blood to the other fetus.  This results in one fetus (donor) with hypovolemia, elevated vasopressin levels, oliguria, and oligohydramnios.  The recipient is volume overloaded and hypertensive.  Hypertension is caused partially by activation of the renin – angiotensin – aldosterone system in the donor.  These substances then cross the placenta and cause hypertension in the recipient.  The recipient also has elevated endothelin-1 contributing to hypertension and increased atrial and brain natriuretic peptide levels leading to polyuria. The recipient twin may develop cardiomyopathy and heart failure manifested by cardiac hypertrophy, cardiac dilatation, systolic dysfunction, A-V valve regurgitation, pulmonary outflow tract obstruction, and hydrops.  Hemodynamic imbalance may impair cerebral perfusion leading to antenatal cerebral injury.  Acute TTTS may result from death of one twin that leads to exsanguination of the living twin into the other twin. The mechanism involves a fall in systemic vascular resistance in the dead fetus. 

Selective IUGR is usually caused by a velamentous or marginal insertion of the cord and/or unequal sharing of the placenta. 

TAPS is caused by a small A-V communication which results in a hematocrit difference between the fetuses, i.e., red baby – white baby.  In the past, this condition was mistakenly thought to be TTTS and required a 5 g/dL difference in hemoglobin for the diagnosis.  TAPS occurs in less than 3% of Mo/Di twins.  It may occur spontaneously, but more often results from laser therapy for TTTS.  Either a small vessel was missed or a vessel reopens after the procedure.  Spontaneous TAPS is usually mild, diagnosed after 30 weeks gestation, and does not require antepartum treatment.  Post laser TAPS can be bad and may require fetal transfusion or repeat laser.  Criteria for postnatal diagnosis of TAPS is Hgb difference >8 g/dL or intertwin reticulocyte ratio of > 1.7 (reticulocyte count of donor divided by reticulocyte count of recipient).

Time Course
<13 weeks: Diagnosis
14-28 weeks: Severe complications – TTTS, early sIUGR, and discordant malformation
30+ weeks: Late complications – late TTTS, late sIUGR, TAPS, and single IUFD

Ultrasound Protocol
10-13 weeks – Diagnose chorionicity and offer first trimester screen
(CRL, heart rate, nuchal translucency, cord insertion, membrane folding)
16-37 weeks – Perform  ultrasound every 2 weeks assessing vertical pockets of amniotic fluid (MVP), visualization of both fetal bladders/stomachs, Doppler studies, and measurement of abdominal circumferences.  Observe for Oli/Poly and EFW <10th percentile.  If apparent discrepancy in amniotic fluid and/or fetal size, but not quite meeting criteria, recheck in 1 week.  Anatomy scan with cord insertions at 18 to 22 weeks.  Cervical lengths at 20 and 24 weeks.  Formal fetal ECHO at 20 to 24 weeks. Full growth scans should be done at least every 4 weeks.
32 weeks and greater – Perform weekly BPPs.  Measure PSV MCA Dopplers every 2 weeks to assess for TAPs. 

Timing of delivery:
Deliver at 36 to 37 weeks to minimize risk of late IUFD. 

Considerations:
Distinguishing TTTS from sIUGR can be difficult.

1. With sIUGR, the normally grown twin will have a normal volume of amniotic fluid (MVP > 2cm and <8cm.)
2. With TTTS, the normally grown twin may have findings of recipient cardiomyopathy.

Fetal death is not strictly related to Quintero stages.  Quintero stages 2, 3, and 4 are usually treated with laser therapy. Caveats concerning the Quintero stages are as follows: 1) onset of TTTS may be gradual or sudden,  2). atypical presentation can occur, e.g., donor twin with both a persistent bladder and abnormal umbilical artery Doppler flow, 3) a particular case may not follow an orderly progression of stages, e.g., a stage 1 case may progress rapidly in a few days to a stage 3, and 4) regression of the disease can occur.  TTTS status post laser is a very high risk situation: 65% survival, 5-20% neurological damage, EGA at delivery 32 weeks, PPROM, amniotic band, abruption. 

In general, single IUFD at < 20 gestational weeks will not result in death or neurological impairment of the remaining twin; however, there are exceptions.  After 20 weeks IUFD of one twin may cause damage usually within 24-48 hours.  Follow with neurosonography (vaginal) and possibly MRI (after 32 weeks gestation).  CNS problems include abnormalities of neural migration and sulcation, PVL, and porencephalic cyst.

References

1. Simpson LL. Twin-twin transfusion syndrome. Am J Obstet Gynecol 2013; 208:13-18.
2. Rychik J, Tian Z, Bebbington M, et al. The twin-twin transfusion syndrome: spectrum of cardiovascular abnormality and development of a cardiovascular score to assess severity of disease. Am J Obstet Gynecol 2007; 392: e1 – e8.
3. Moise KJ, Johnson A. Pathogenesis and diagnosis of twin-twin transfusion syndrome. UpToDate. Last updated 12-2-13. Accessed 1-7-14.