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Purpose: In assisted reproductive technology, normal zygotes are bipronuclear (2PN) during fertilization confirmation; however, sometimes, nonpronuclear zygotes (0PN) and monopronuclear zygotes (1PN) are found during routine observations. Methods: To elucidate the clinical usefulness of in vitro‐fertilized embryos, we investigated the rates of clinical pregnancy, live birth, miscarriage, and congenital abnormality after transfer of frozen‐thawed 1PN‐ and 0PN‐derived single blastocysts at Denentoshi Ladies Clinic, Kanagawa, Japan. Results: The rates of pregnancy and live birth for 1PN‐derived blastocysts obtained by conventional in vitro fertilization were 37.5% and 27.1%, respectively, which was not significantly different from those for 2PN‐derived blastocysts; however, the rates for 0PN‐derived blastocysts were significantly lower. The pregnancy and live birth rates for 0PN‐derived embryos obtained by intracytoplasmic sperm injection (ICSI) were 45.7% and 34.8%, respectively, which was not significantly different from those for 2PN‐derived blastocysts; however, the rates for 1PN‐derived blastocysts were significantly lower (4.0% for both) than those for 2PN‐ and 0PN‐derived blastocysts. No congenital abnormalities were found in infants resulting from transfer of 0PN‐ or 1PN‐derived blastocysts. Conclusions: Both 1PN‐ and 0PN‐derived blastocysts can be used for embryo transfer; however, care should be taken in making decisions about 1PN‐derived blastocysts, especially if they are obtained by ICSI. [ABSTRACT FROM AUTHOR]