Biparietal diameter in twins at gestational weeks 18-32. Differences and similarities. Le Groupe Romulus.

OBJECTIVE: To examine twin similarities on biparietal diameter (BPD) measurements by zygosity (monozygotic [MZ] and dizygotic [DZ]) and chorionicity (monochorionic [MC] and dichorionic [DC]) and their evolution during pregnancy.

METHOD: A sample of 54 pairs of twins (43 DZ sets, 11 MZ sets [7 MC and 4 DC]) was constructed using retrospective data. Despite the small sample size, our data were complete, and, for the first time we measured different fetal parameters on digital ultrasound images outside routine examination. The intraexaminer and interexaminer reliability of BPD measurement was significant (r = .95, P < .001).

RESULTS: In this study, developmental results indicate significant linear regression coefficient (R) through the whole period of gestation (r = .96, P < .001), though product moment correlations comparing the periods of gestation two by two are weaker. The distribution of BPD values was slightly wider at the 28th week and markedly wider at the 32nd week than that at the 18th and 23rd weeks. The intraclass correlations of DZ and MZ (MC and DC) twins were examined at the 18th, 23rd, 28th and 32nd weeks of amenorhea. The intraclass correlations of DZ twins were significant through the whole period of gestation (r = .45, P = .001; r = .27, P = .04; r = .36, P = .008; and r = .42, P = .002, respectively), whereas the intraclass correlations of MZ twins were significant only at the 18th, 23rd and 28th weeks (r = .73, P = .002; r = .69, P = .005; r = .49, P = .047, respectively). We found significant within-variance differences not only between DZ and DC-MZ but also between DC-MZ and MC-MZ in late gestation.

CONCLUSION: Our analysis of twin BPD development demonstrated that zygosity and chorionicity type are both important determinants of twin fetal development.