Causes of Slow Fetal Growth in the Third Trimester

A fetus develops on a specific timetable, with the third trimester devoted mostly to gaining weight. In 3 to 10 percent of pregnancies, fetal growth lags in the last few months of pregnancy, a condition known as intrauterine growth retardation or restriction -- IUGR. This condition increases the risk of fetal death before or around the time of delivery. Many factors contribute to IUGR.

Maternal Factors

The main maternal cause of IUGR is high blood pressure, causing around one third of all cases of IUGR. Other maternal factors associated with IUGR include kidney disease, diabetes, lung or heart disease. Cigarette smoking during pregnancy may cause IUGR, and the risk increases the more a mother smokes. Alcohol consumption, poor diet, low maternal weight and poor weight gain also are implicated in IUGR, along with young maternal age, poverty, recent pregnancy and high number of previous births.

Placental Factors

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The placenta supplies nutrients to the fetus and removes waste products, so diseases that interfere with its functioning, such as high blood pressure, diabetes and kidney disease can cause IUGR. Other causes of decreased blood flow include a placenta that implants too low on the uterine wall, part or all of the placenta detaching prematurely from the uterine wall and infection.

Fetal Factors

Genetic and chromosomal defects can cause IUGR in a fetus. Fetal exposure to infections, which include toxoplasmosis, rubella, cytomegalovirus and herpes simplex or varicella can lead to IUGR. Prenatal infections often have a poor long term prognosis. Multiple pregnancies also increase the risk of IUGR -- identical twins who share a placenta develop twin to twin transfusion syndrome, where one twin receives too much of the blood supply and one receives too little, developing IUGR.