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Deformities in the Fetus

By Susan T. McClure ; Updated June 13, 2017

After the eighth week of pregnancy, your developing baby is properly called a fetus. By this time, all of the major organs and structures have formed. According to “Your Pregnancy Week by Week,” only 3 percent of all babies are born with major birth defects and most of those developed in the first 13 weeks of pregnancy.


Fetal deformities can occur from genetic abnormalities. Sometimes a fetus has an extra chromosome, a situation called trisomy. Prescription medications like the blood thinner coumadin or valium can cause structural defects or miscarriage. Exposure to glue or solvents can deform the face and stunt the growth of the head. The fetus of a pregnant woman who uses cocaine can have deformities in the genitals and urinary tract, limbs, face, heart and bowels. Fetal alcohol syndrome causes, among other problems, heart defects and abnormalities in the limbs and face. Smoking and even exposure to secondhand smoke during pregnancy can cause neural tube defects like spina bifida. However, most fetal deformities occur with no clear identifiable cause.


According to “Your Pregnancy Week by Week," heart defects are the most common structural defects, affecting one in 125 newborns. Polydactyly and sydactyly are minor defects of extra fingers or toes and fused fingers or toes, respectively. Cleft lip and cleft palate occur when the sides of the roof of the mouth fail to fuse during development. The neural tube defect spina bifida occurs when the spinal cord protrudes out of the bony spinal column, potentially causing paralysis. Some fetuses with an extra copy of chromosome 21 (Down’s syndrome), 18 (Edward’s syndrome) and 13 (Patau syndrome) survive until birth but have a wide range of physical deformities.


Tests of maternal blood can identify a risk of Down’s syndrome, spina bifida, defects in the abdominal wall of fetus and anencephaly--absence of the skull, usually fatal a few days after birth.

Ultrasound at 10 to 13 weeks of pregnancy to measure nuchal translucency--abnormal fluid collection behind the baby’s neck and the nasal bone--can predict Down’s sydrome in 95 percent of cases, according to “Your Pregnancy Week by Week.” If a doctor suspects abnormalities of the hands or feet, spine or face she might recommend three-dimensional ultrasound.

Invasive tests of fetal cells called amniocentesis and chorionic villus sampling (CVS) identify genetic abnormalities.


Severe fetal deformities usually trigger an early miscarriage. About 20 percent of all pregnancies end in a miscarriage, often before a woman realizes she was pregnant, says “Pregnancy for Dummies.” Half of all miscarriages occur because of a chromosomal defect. In 20 percent of miscarriages, the embryo or fetus has a structural defect that doctors cannot detect with ultrasound. Other outcomes depend on the exact nature of the defects and often cannot be predicted accurately before birth.

Risk of Recurrence

The risk that you will have another pregnancy resulting the same fetal deformity depends on the cause of the first one, says “Pregnancy for Dummies." If the doctor cannot find the cause of a fetal deformity, the risk of recurrence is low. If the cause is genetic, a genetic counselor can determine the risk of recurrence.

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