Reference values for differences between cardio-circulatory variables of normal twin fetuses

Background No reference values exist concerning the differences between car dio-circulatory variables of normal twin fetuses. The normal data could be useful in the identification of conditions causing opposite hemodynamic eff ects on each twin. Objective To establish the normal differences among cardio-circulatory para meters of twin fetuses during the second and third trimesters of gestation. Materials and methods Twenty-seven normal twin pregnancies were used in thi s longitudinal and prospective study. Doppler-echocardiographic investigati ons were performed every 2-3 weeks starting at an average of 23.11 +/- 3.13 (mean +/- standard deviation) weeks' gestation. At each visit, the followi ng cardio-circulatory variables were evaluated: the cardio-thoracic ratio, the ventricular wall and septal thicknesses, end-diastolic and systolic dia meters, ventricular fractional shortenings, velocity of circumferential fib re shortenings and left and right ventricular outputs. In addition the foll owing measurements were made from Doppler recordings: through both aortic a nd pulmonary valve the acceleration and ejection times, the peak systolic v elocities and the velocity time integrals; and through both mitral and tric uspid valves peak velocities of E and A waves and the EIA ratios. Finally t he pulsatility index of the umbilical artery was also evaluated. Results There was no single variable where the intertwin difference changed with gestational age. No systematic difference between the smaller and lar ger twin could be demonstrated for any variables except for the cardiothora cic ratio. Conclusion Reference tables should permit a comparative approach between th e two twins in the investigation of life threatening complications such as twin-to-twin transfusion syndrome.