Birth Defects

Symptoms and Causes

What causes birth defects?

Experts don’t know the exact cause of most birth defects, but some reasons are:

• Genetic or hereditary factors.
• Infection during pregnancy.
• Drug exposure during pregnancy.

How do genes and chromosomes affect birth defects?

About 20% of birth defects are caused by genetic or hereditary factors. Genetic causes of birth defects fall into three general categories:

• Chromosomal abnormalities.
• Single-gene defects.
• Multifactorial.

Every human body cell contains 46 chromosomes, and each chromosome contains thousands of genes. Each gene contains a blueprint that controls development or function of a particular body part. People who have either too many or too few chromosomes will therefore receive a scrambled message regarding anatomic development and function.

Down syndrome is an example of a condition caused by too many chromosomes. Because of an accident during cell division, individuals with Down syndrome have an extra copy of a particular chromosome (chromosome 21). This extra chromosome can cause a typical constellation of birth defects. Characteristic features of Down syndrome can include developmental delay, muscle weakness, downward slant of the eyes, low-set and malformed ears, an abnormal crease in the palm of the hand and birth defects of the heart and intestines.

With Turner syndrome, a female lacks part or all of one X chromosome. In the affected women, this can cause short stature, learning disabilities and absence of ovaries.

Trisomy 13 (Patau Syndrome) and Trisomy 18 (Edwards Syndrome) are caused by inheriting extra copies of the 13th or 18th chromosome, respectively. These are rarer, more serious conditions which cause severe birth defects that are incompatible with survival after birth.

In addition to inheriting an extra or absent chromosome, deletions or duplications of single genes can also cause developmental disorders and birth defects. One example is cystic fibrosis (a disorder that causes progressive damage of the lungs and pancreas).

Defective genes can also be caused by accidental damage, a condition called “spontaneous mutation.” Most cases of achondroplasia (a condition that causes extreme short stature and malformed bones) are caused by new damage to the controlling gene. In addition, recombination errors can cause translocations of chromosomes which may lead to complex birth defects.

How do environmental factors like infection and drugs affect birth defects?

Environmental factors can increase the risk of miscarriage, birth defects, or they might have no effect on the fetus at all, depending on at what point during the pregnancy the exposure occurs.

The developing fetus goes through two major stages of development after conception. The first, or embryo stage, occurs during the first 10 weeks after conception. Most of the major body systems and organs form during this time. The second, or fetal stage, is the remainder of the pregnancy. This fetal period is a time of growth of the organs and of the fetus in general. Your developing fetus is most vulnerable to injury during the embryo stage when organs are developing. Indeed, infections and drugs can cause the greatest damage when exposure occurs two to 10 weeks after conception.

Diabetes and obesity can possibly increase your child’s risk of birth defects. Your healthcare provider may suggest that you do your best to manage these conditions before you get pregnant.

Some medicines and recreational drugs can cause birth defects, which are most severe when used during the first three months of pregnancy. Thalidomide, an anti-nausea medicine prescribed during the 1960s, caused birth defects called phocomelia (absence of most of the arm with the hands extending flipper-like from the shoulders).

There’s also isotretinoin, previously known as Accutane or Roaccutane. Isotretinoin is a retinoid, a man-made form of vitamin A that’s used to treat skin conditions. It causes fetal retinoid syndrome. Characteristics of fetal retinoid syndrome may include:

• Growth delay.
• Malformations of the skull and face.
• Abnormalities of the central nervous system.
• Abnormalities of the heart.
• Abnormalities of the parathyroid gland.
• Abnormalities of the renal, thymus gland.

Alcohol is the most commonly used drug that causes birth defects. Fetal alcohol syndrome is a term used to describe the typical birth defects caused by the pregnant person's alcohol use:

• Learning disabilities.
• Developmental delay.
• Irritability.
• Hyperactivity.
• Poor coordination.
• Abnormalities of facial features.

Another environmental factor that can cause birth defects is uterine constraint. The fetus grows in the uterus and is surrounded by amniotic fluid (similar to being suspended in a bag of water) that cushions it from excessive pressure. If the sack of fibers that holds the fluid breaks, bands of fibers from the torn sack can press on the fetus and cause amniotic band syndrome (which can result in partial contraction of or amputation of an arm or leg). An inadequate amount of amniotic fluid can cause excessive pressure on the fetus, causing pulmonary hypoplasia (lack of development of the lungs).

Medical science has identified the cause of about 30% of birth defects. That means about 70% remain without a straightforward cause. As many as 50% to 70% of birth defects are sporadic, and their cause remains unknown. A combination of environmental and genetic factors can increase the risk of certain birth defects.