[Reference Birthweight for Multiple Births in Japan]

Purpose: Intrauterine growth curves of twins, that is, birth weights according to gestational age, were calculated from birth certificate data.

Methods: Multiple births were identified by birthplace, ages of the parents, gestational age, and year and month of birth. There were 49,240 twin births in Japan between 1988 and 1991. Of these, 32,232 livebirth-livebirth pairs, 679 livebirth-stillbirth pairs, and 278 stillbirth-livebirth pairs were included in this analysis. There were also 1894 triplet live births from 744 sets of triplets and 206 quadruplet live births analyzed. For all, access was made to the database of birth certificates in the form of magnetic tapes giving birthweights in hundred gram categories.

Results: For all gestational ages, median birthweights of males were ca. 0.05 kg-0.1 kg larger than female values. Compared to singleton births in Japan, median birthweights of twins remained ca. 0.15 kg smaller until gestational age of 34 weeks, the difference then increasing to ca. 0.5 kg at 42 weeks of gestation. As for birth order, mean birthweight of the first twin was larger than that of the second and the standard deviation was larger for the second. Birthweights of twins from multiparous mothers were greater than those from primiparous mothers. Among the multiple births, median birthweight for gestational age was found to be greatest in twins, lower in triplets and lowest in quadruplets. In triplets, the 50th centile for boys was 0.08 kg larger than for girls.

Discussion: With regard to perinatal growth, the fetus is affected more or less by the limitation of uterine expansion in the late gestational weeks. Reference birthweights for exclusive use for multiple births are different from that for singletons.