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Objective: To explore whether there exist differences in cognitive development between singletons and twins born after in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI)., Methods: A total of 566 children were recruited for the study, including 388 children (singletons, n=175; twins, n=213) born after IVF and 178 children (singletons, n=87; twins, n=91) born after ICSI. The cognitive development was assessed using the Chinese-Wechsler Intelligence Scale for Children (C-WISC)., Results: For all pre-term offspring, all the intelligence quotient (IQ) items between singletons and twins showed no significant differences no matter if they were born after IVF or ICSI. There was a significant difference in the cognitive development of IVF-conceived full-term singletons and twins. The twins born after IVF obtained significantly lower scores than the singletons in verbal IQ (containing information, picture & vocabulary, arithmetic, picture completion, comprehension, and language), performance IQ (containing maze, visual analysis, object assembly, and performance), and full scale IQ (P<0.05). The cognitive development of full-term singletons and twins born after ICSI did not show any significant differences. There was no significant difference between the parents of the singletons and twins in their characteristics where data were collected, including the age of the mothers, the current employment status, the educational backgrounds, and areas of residence. There were also no consistent differences in the duration of pregnancy, sex composition of the children, age, and height between singletons and twins at the time of our study although there existed significant differences between the two groups in the sex composition of the full-term children born after ICSI (P<0.05)., Conclusions: Compared to the full-term singletons born after IVF, the full-term twins have lower cognitive development. The cognitive development of full-term singletons and twins born after ICSI did not show any significant differences. For all pre-term offspring, singletons and twins born after IVF or ICSI, the results of the cognitive development showed no significant differences.