Pregnancy order. A factor influencing birth weight in twin gestations.

To test the hypothesis that increased twin birth weight is expected with increased parity, we conducted a retrospective, cross-sectional evaluation of 430 twins, focusing on birth weight and growth discordance parameters in five parity groups (para 1-4 and > or = 5). The mean birth weight of twin A, B and both twins was significantly lower in primiparous patients as compared to para 2-4 but not as compared to para > or = 5 patients. The frequency of twins weighing < 1,500 g was similar in all groups, but a significantly higher frequency of twins weighing 1,500-2,500 g and lower frequency at > 2,500-g twins were found in the para 1 group as compared to the groups with higher parities. Presentation combinations were not different in the five parity groups, nor were the frequencies of growth discordance categories. However, discordants among pairs, with each infant weighing < 2,500 g, occurred almost twice as often in para > or = 5 as compared to the other parity groups. Our data indicate that primiparae seem to have significantly smaller twins as compared to multiparae. This trend was independent of the presentation combinations and growth discordance pattern and seems to result from a higher frequency of twins weighing 1,500-2,500 g in primiparae.