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

Background No reference values exist concerning the differences between cardio-circulatory variables of normal twin fetuses. The normal data could be useful in the identification of conditions causing opposite hemodynamic effects on each twin. Objective To establish the normal differences among cardio-circulatory parameters of twin fetuses during the second and third trimesters of gestation. Materials and methods Twenty-seven normal twin pregnancies were used in this longitudinal and prospective study. Doppler-echocardiographic investigations were performed every 2-3 weeks starting at an average of 23.11 +/- 3.13 (mean +/- standard deviation) weeks' gestation. At each visit, the following cardio-circulatory variables were evaluated: the cardio-thoracic ratio, the ventricular wall and septal thicknesses, end-diastolic and systolic diameters, ventricular fractional shortenings, velocity of circumferential fibre shortenings and left and right ventricular outputs. In addition the following measurements were made from Doppler recordings: through both aortic and pulmonary valve the acceleration and ejection times, the peak systolic velocities and the velocity time integrals; and through both mitral and tricuspid valves peak velocities of E and A waves and the EIA ratios. Finally the 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 larger twin could be demonstrated for any variables except for the cardiothoracic ratio. Conclusion Reference tables should permit a comparative approach between the two twins in the investigation of life threatening complications such as twin-to-twin transfusion syndrome.