Objective: The goal of this study has been to determine frequency of occurrence of monochorial twins within the study group consisting of 521 pregnancies conceived through the In Vitro Fertili- zation (IVF) programme. Design: An observational study. Setting: Sanatorium Pronatal, Na Dlouhé mezi 4/12, 147 00 Praha 4 - Hodkovičky. Methods: A condition to be included in the study group was that there was a gestation sac detec- ted by ultrasound. The first ultrasound examination was performed transvaginally and was done between the fifth and the sixth week of each pregnancy. A multiple pregnancy has been classified as monochorial in case when gestation sac contains two yolk sacs and two fetuses. Results: Within the whole study group there were 13 monochorial twins which presents 2.5% of all pregnancies. The incidence of monochorionicity among all multiple pregnancies is 6,4%, in IVF cycles without micromanipulation techniques the occurence of monochorial twins has been 0.9%, following cryoembryotransfer 3.9%, with micromanipulation (ICSI, AH) it reaches 5.7% of all pregnancies. Conclusion: Monochorial twins were detected seven time more frequent in comparison within occurence in ordinary population. The occurence rises in connection with the number of fetuses in uterus. In comparison with simple IVF cycles we have proved higher occurence of monochorio- nicity in connection with micromanipulation techniques and cryoembryotransfer.

        Key words: monochorial twins, in vitro fertilization, micromanipulation techniques