If one gives birth to a premature/preterm son, or if the childs weight is to low, for example, would it still be circumcised on the 8th day or are there exceptions to the command?
If it is a matter of the health of the infant, then the bris is delayed until he is "recovered" from whatever is wrong. It is not an automatic situation but determined by the mohel and the doctor.
A Bris Milah is never performed if it poses any danger to the infant. The doctor and/or mohel's advice to delay a Bris for health reasons should always be heeded. In case of jaundice (yellow pigmentation of the skin), the bris cannot be performed; it is delayed until the bilirubin in the blood drops to a safe level. The bris may then be performed without endangering the child. In cases of some illnesses, a delay of seven days following full recovery is required. It is forbidden to postpone the bris for any reason other than health of the child, or in order to obtain a proper Mohel. It is the responsibility of the Mohel, in consultation with the doctor and the family, to determine if a delay is necessary according to Jewish law.