My sister is amlost 5 months pregnant and started smoking again. what can happen to her unborn baby?

IUGR: 2 big problems with smoking during pregnancy: decreased placental blood flow resulting in decreased nutrients and oxygenation for the baby resulting in restricted growth (intrauterine growth restriction). Other problem is increased risk of placental abruption (shearing away of the placenta from the uterine wall) which can result in catastrophic outcome for the baby and mother.

Don't smoke: Low birth weight, lower IQ possibilty.

Increased problems: The infant of a smoker will see their placental blood vessels age faster & start to shrink, reducing their blood flow & cutting their birth weight by 1/2 lb or more. They will have a higher risk of crib death, 2-3x as many ear & lung infections. They will grow up wanting to smoke because their source people do. Interesting legacy to give someone you are supposed to love.