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J Ultrasound Med. 2001 Jul;20(7):757-60; quiz 761.

Frequency of a monochorionic pair in multiple gestations: relationship to mode of conception.

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Department of Radiology, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts 02115, USA.



To determine whether the mode of conception affects the frequency of monochorionicity in multiple gestations.


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 the frequency in pregnancies achieved via any form of assisted reproductive technology and among the different types of assisted reproductive technology.


Our study consisted of 464 multiple gestations comprising 332 twin, 113 triplet, 16 quadruplet, and 3 quintuplet pregnancies. The higher the fetal number, the more likely the pregnancy resulted from assisted reproductive 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 resulting from assisted reproductive technology (28.2% versus 5.4%; P < .000001, chi2 test). The frequency of monochorionic pairs after in vitro fertilization with blastocyst transfer on day 5 (10.5%) was double the frequency from in vitro fertilization with cleavage stage transfer on day 3 (4.9%), but the difference was not statistically significant (P = .24, Fisher exact test).


Monochorionic pairs are relatively common in naturally conceived twins and in higher-order multiple gestations with more than 3 fetuses arising from assisted reproductive technology, but they are uncommon in twins and triplets arising from assisted reproductive technology There 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 needed to confirm this finding.

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