A premature baby:[1]

Based on standard medical advice, it is suggested to always wait until the 36-37th week prior to circumcising, even if the child is Halachically allowed to be circumcised.

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[1] Otzer Habris 8:27; Piskeiy Teshuvos 331:4; Pesakim Uteshuvos 262:10; See Kaf Hachaim 331:2; Minchas Yitzcha 4:123; Pesakim Uteshuvos 262:10 footnotes 79-82; See Igros Moshe Yoreh Deah 2:121 that if a child is in an incubator, he is considered ill and cannot be circumcised until 7 days have passed after he is removed.