Objective: The aim of this study was to evaluate placental abnormalities in relation to birth weight discordance in dichorionic and monochorionic twins.
Study design: The maternal charts and placental abnormalities of 147 structurally normal twin pairs with cords labeled at delivery were reviewed. The placental weight belonging to each twin was determined by measuring the length, width, and thickness in each of the two placental disks. Placental weight, chorionicity, infarction, abruptio placentae, decidual vascular abnormality, villous fibrosis and hypovascularity, chronic villitis, and intraplacental thrombi were also assessed. Birth weight was discordant if > or = 20%. The data were analyzed with chi 2 and analysis of variance after log transformation of skewed discordancy values.
Results: Of the 147 twin pairs, 99 were dichorionic and 48 monochorionic. Placental weights were known for 91 dichorionic and 40 monochorionic twins. Of the lighter cotwins in dichorionic twin pairs 36.3% (33/91) belonged to the heavier placenta, 49.5% (45/91) belonged to the lighter placenta, and 14.3% (13/91) had an equal share of the placental weight with the heavier sibling (p < 0.05). In 42.4% (42/99) the lighter dichorionic twin had more placental lesions than the heavier twin, in 38.4% (38/99) the same number of lesions were present in both placentas, and in 19.2% (19/99) the heavier twin had more placental lesions. There was linear correlation between percent discordance and number of placental lesions in the lighter twin. In dichorionic twins 18 of the 99 (18.1%) were discordant. In 77.8% (14/18) the lighter twin had more placental lesions than the heavier twin, in 16.7% (3/18) the number of lesions was the same in both, and in 5.6% (1/18) the heavier twin had one more lesion than the lighter twin (p < 0.05). In monochorionic twins, regardless of birth weight discordance, no differences in placental abnormalities were observed.
Conclusions: In dichorionic twins significant birth weight discordance was attributable not to differences in placental weight but to a greater number of placental lesions in the lighter twin than in the heavier twin (p < 0.05). This did not hold true for monochorionic twins.