Doctors think that the two types of IUGR may be linked to the time during development that the problem occurs. Symmetrical IUGR may occur when the unborn baby experiences a problem during early development. Asymmetrical IUGR may occur when the unborn baby experiences a problem during later development. While not true for all asymmetrical cases, doctors think that sometimes the placenta may allow the brain to get more oxygen and nutrition while the liver gets less.

There are many IUGR risk factors involving the mother and the baby. A mother is at risk for having a growth restricted infant if she:

• Has had a previous baby who suffered from IUGR
• Is small in size
• Has poor weight gain and nutrition during pregnancy
• Is socially deprived
• Uses substances (like tobacco, narcotics, alcohol) that can cause abnormal development or birth defects
• Has a vascular disease (like preeclampsia)
• Has chronic kidney disease
• Has a low total blood volume during early pregnancy
• Is pregnant with more than one baby
• Has an antibody problem that can make successful pregnancy difficult (antiphospholipid antibody syndrome).

Additionally, an unborn baby may suffer from IUGR if it has:

• Exposure to an infection, including German measles (rubella), cytomegalovirus, tuberculosis, syphilis, or toxoplasmosis
• A birth defect (like a severe cardiovascular defect)
• A chromosome defect, especially trisomy 18 (Edwards' syndrome)
• A primary disorder of bone or cartilage
• A chronic lack of oxygen during development (hypoxia)
• Placenta or umbilical cord defects
• Developed outside of the uterus.