Final answer:
Poor nutrition is identified as the major factor in low birthweight, affecting the baby's susceptibility to diseases and potential developmental issues. Smoking and drug use are also significant contributors but primary focus on maternal nutrition is crucial.
Step-by-step explanation:
The major factor in low birthweight is poor nutrition. Maternal undernutrition leads to intrauterine growth restriction, which is a significant cause of low birthweight in infants. A low birth weight (< 2.5 kg, or 5.5 lb.) due to poor nutrition makes babies more susceptible to diseases and increases the likelihood of infant mortality. If they survive, it can lead to slow growth and developmental delays.
It's important to note, however, that factors such as smoking, drug use during pregnancy, and heredity also contribute significantly to low birthweight. Smoking while pregnant, in particular, reduces the blood oxygen levels for the developing baby, which can result in complications including low-birth-weight infants. Nevertheless, given that poor nutrition affects both the mother's health and the in-utero environment directly, it is considered the major factor.