DOPPLER VELOCIMETRY DETERMINED REDISTRIBUTION OF FETAL BLOOD-FLOW - CORRELATION WITH GROWTH RESTRICTION IN DIAMNIOTIC MONOCHORIONIC AND DIZYGOTIC TWINS

OBJECTIVE: Our purpose was to study fetal growth and blood flow distri bution in diamniotic monochorionic compared with dizygotic (diamniotic dichorionic) twins by use of Doppler velocimetry of the umbilical art ery and middle cerebral artery. STUDY DESIGN: Study candidates were di vided into group A, consisting of 33 pairs (66 fetuses) of diamniotic monochorionic twins, and group B, 50 pairs (100 fetuses) of diamniotic dichorionic twins. Diamniotic monochorionic placentation was confirme d by microscopic placental examination for group A. Diamniotic dichori onic placentation was ensured for group B by selecting only twins with different-sex pairs (dizygotic twins).Targeted ultrasonography with b iometry was performed in each twin, and Doppler recordings of the umbi lical artery and middle cerebral artery were obtained. Waveforms were analyzed and the systolic/diastolic ratio, the resistance index, and a measure of blood flow redistribution (brain-sparing effect), the cere bral/placental ratio, was calculated for each fetus. Growth status at birth was assessed by the number of small-for-gestational-age infants (less than or equal to 10th percentile), low-birth-weight infants (les s than or equal to 25th percentile), and percent of growth discordance between twins. Intertwin differences were assessed by Delta values (v alue of larger twin minus value of smaller twin). RESULTS: Diamniotic monochorionic compared with dizygotic twins demonstrated a significant ly greater probability of blood flow redistribution. For the study pop ulation as a whole, the brain-sparing effect was noted in 67% of small -for-gestational-age babies and only 7% of non-small-for-gestational-a ge infants (p less than or equal to 0.001). For the diamniotic monocho rionic pregnancies blood flow redistribution occurred in 6 of 10 small -for-gestational-age infants (60%) and 6 of 46 non-small-for-gestation al-age infants (13%). In the diamniotic monochorionic group small-for- gestational-age compared with non-small-for-gestational-age infants we re more likely to show blood flow redistribution, which was the result of significantly decreased resistance in the middle cerebral artery a nd significantly increased resistance in the umbilical artery. Small-f or-gestational-age infants (less than or equal to 10th percentile) occ urred much less frequently in the dizygotic group. Two of two small-fo r-gestational-age infants in the dizygotic group showed blood flow red istribution. Although the extremes of birth weight were more common in the diamniotic monochorionic group, both groups had relatively large numbers of small babies with birth weights in the lower 25th percentil e (50.0% for diamniotic monochorionic and 44.0% for dizygotic twins, n ot significant). However, 42.3% (11/26) of diamniotic monochorionic tw ins who were in the low-birth-weight group showed blood flow redistrib ution compared with only 3.3% (1/30) whose birth weights were greater than or equal to 25th percentile (p less than or equal to 0.001). In t he dizygotic twins 10% of lower-birth-weight infants redistributed blo od flow compared with 1% in the higher-birth-weight group, a nonsignif icant difference. Diamniotic monochorionic compared with dizygotic twi ns were delivered earlier (32.9 weeks vs 34.8 weeks, p less than or eq ual to 0.001), were smaller (1832 gm vs 2304 gm, p less than or equal to 0.001), showed higher birth weight discordance (29.8% vs 14%, p les s than or equal to 0.05), and had greater numbers (19.7% vs 2.3%, p le ss than or equal to 0.01) of infants at less than or equal to 10th per centile birth weight. CONCLUSIONS: Diamniotic monochorionic twins from the lower-birth-weight groups more often show blood flow redistributi on compared with dizygotic twins of similar low birth weights. Placent al vascular connections and the attendant hemodynamic changes in the f etuses of diamniotic monochorionic twins probably account for this dif ference. Brain-sparing events occur commonly without clinical twin tra nsfusion syndrome in this group. These findings have implications for management.