Causes of low birth weight can be classified into different categories, depending on their contribution to the low birth weight.

A. Fetal:

Fetal distress occurs during any abnormality on labour and delivery causing the baby to stay long after entering into the birth canal as in obstructed labour due to cephalo-pelvic disproportion, abnormal positioning of the fetus (occipital posterior position), abnormal presentations of the fetus (breech , face, shoulder and brow presentation).
Multiple pregnancies: this means the pregnancy of more than one fetus. In this case, unless the maternal nutritional supply is enough, the babies will compete for nutrients, and one will get enough, and other may suffer from lack of nutrients which deprive the normal growth of the baby and become low in his birth weight when delivered.
Erythroblastosis fetalis: In this case, the number of red blood cells will decrease due to the destruction of the RBCs caused by blood group incompatibility; which will lead to decreased supply of oxygen to the cells and slow cellular multiplication causing immaturity on the baby.

B. Placental:

Premature separation(abruption placenta): The placenta will separate from the endometrial wall before the right time for separation, and this will cause nutritional deprivation to the fetus and results in the immaturity of the fetus.
Placenta previa: This is an abnormal situation of the placenta causing a disturbance in the blood supply to the fetus which in turn causes the lower supply of nutrients.

C. Uterine: Incompetent cervix: This is a condition in which the cervix loses its muscular strength, letting the baby out before the right time.

D. Maternal: