Objective: To challenge the hypothesis that discordant growth is a normal variation by relating birth weight discordance to total twin birth weight. METHODS Among 12,565 Israeli live-born twin pairs (1993-98), we compared total twin birth weight decile, the frequencies of three levels of discordance in the general population, over 25% discordance between like- versus unlike-sex pairs, and over 25% discordance between pairs delivered by primiparas 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, respectively). The best-fit correlation of frequencies of more than 25% discordance was inversely logarithmic and similar in like- and unlike-sexed twins across deciles functions (P =.7, odds ratio [OR] 1.0, 95% confidence interval [CI] 0.9, 1.2). The overall frequencies of discordance were also similar (9.3% versus 10.2%, P =.11, OR 0.9, 95% CI 0.8, 1.0). The frequencies of primiparas decreased linearly (R(2) =.98) and the frequencies of more than 25% discordant pairs in multiparas and primiparas across the deciles had similar inversely logarithmic patterns (P =.55, OR 1.0, 95% CI 0.9, 1.2). Discordance over 25% was significantly more frequent among primiparas (P <.001, OR 1.45, 95% CI 1.3, 1.6).
Conclusion: The observed patterns of birth weight discordance did not substantiate normal variation but an adaptive growth restriction that might explain why the likelihood of discordant growth decreases as total twin birth weight increases.