Adaptive growth restriction as a pattern of birth weight discordance in twin gestations

Objective: To challenge the hypothesis that discordant growth is a normal v ariation by relating birth weight discordance to total twin birth weight. Method: Among 12,565 Israeli live-born twin pairs (1993-98), we compared to tal twin birth weight decile, the frequencies of three levels of discordanc e in the general population, over 25% discordance between like- versus unli ke-sex pairs, and over 25% discordance between pairs delivered by primipara s versus multiparas. Results: We found a marked change in the best-fit correlation function with increased discordance: level 15-24.9% was inversely linear whereas levels 25-34.9% and over 34.9% were inversely logarithmic (R-2 =.47,.88, and .9, r espectively). The best-fit correlation of frequencies of more than 25% disc ordance was inversely logarithmic and similar in like- and unlike-sexed twi ns across deciles functions (P =.7, odds ratio [ORI 1.0, 95% confidence int erval [CI] 0.9, 1.2). The overall frequencies of discordance were also simi lar (9.3% versus 10.2%, P =.11, OR 0.9, 95% CI 0.8, 1.0). The frequencies o f primiparas decreased linearly (R-2 =.98) and the frequencies of more than 25% discordant pairs in multiparas and primiparas across the deciles had s imilar inversely logarithmic patterns (P =.55, OR 1.0, 95% CI 0.9, 1.2). Di scordance over 25% was significantly more frequent among primiparas (P <.00 1, OR 1.45, 95% CI 1.3, 1.6). Conclusion: The observed patterns of birth weight discordance did not subst antiate normal variation but an adaptive growth restriction that might expl ain why the likelihood of discordant growth decreases as total twin birth w eight increases. (Obstet Gynecol 2000;96:986-90. (C) 2000 by The American C ollege of Obstetricians and Gynecologists.).