Frequency of a monochorionic pair in multiple gestations - Relationship tomode of conception

Objective. To determine whether the mode of conception affects the frequenc y of monochorionicity in multiple gestations. Methods. Our study population consisted of all women with multiple gestations who had a first-trimester sonogram at our institution between May 1998 and April 2000. The frequency of monochorionicity in pregnancies conceived naturally was compared with th e frequency in pregnancies achieved via any form of assisted reproductive t echnology and among the different types of assisted reproductive technology . Results. Our study consisted of 464 multiple gestations comprising 332 tw in, 113 triplet, 16 quadruplet, and 3 quintuplet pregnancies. The higher th e fetal number, the more likely the pregnancy resulted from assisted reprod uctive technology (72.6% of twins, 84.1% of triplets, 92.8% of quadruplets, and 100% of quintuplets; P < .05, Fisher exact test). Monochorionic pairs were found more commonly in naturally conceived pregnancies than in those r esulting from assisted reproductive technology (28.2% versus 5.4%; P < .000 001, chi (2) test). The frequency of monochorionic pairs after in vitro fer tilization with blastocyst transfer on day 5 (10.5%) was double the frequen cy from in vitro fertilization with cleavage stage transfer on day 3 (4.9%) , but the difference was not statistically significant (P = .24, Fisher Exa ct test). Conclusions. Monochorionic pairs are relatively common in natural ly conceived twins and in higher-order multiple gestations with more than 3 fetuses arising from assisted reproductive technology, but they are uncomm on in twins and triplets arising from assisted reproductive technology. The re is a trend toward a higher frequency of monochorionic pairs after day 5 blastocyst transfer than day 3 transfer but a larger study population is ne eded to confirm this finding.