According to the March of Dimes, about 1 in 33 babies born each year in the United States has a birth defect.

That's why experts recommend that women planning to become pregnant visit their doctor first. This is especially important for women with chronic health conditions, the March of Dimes says.

Cleft lip and cleft palate are birth abnormalities of the mouth and lip. They happen in about one in 700 births. These abnormalities occur early in pregnancy when the sides of the lip and the roof of the mouth do not fuse together as they should. A child can have cleft lip, cleft palate, or both. Most babies born with a cleft are otherwise healthy with no other birth abnormalities. Down syndrome is a genetic disorder that includes a combination of birth defects, including some degree of mental retardation, characteristic facial features and, often, heart defects, visual and hearing impairment, and other health problems. These problems vary greatly in severity. Down syndrome affects approximately one in 800 to 1,000 babies.

Smoking increases the risk for miscarriage or damage to developing fetal organs. Alcohol and illegal drugs can increase the risk for fetal abnormalities and miscarriage.

If a woman is not immune to rubella, infection during pregnancy can result in a group of birth defects called the congenital rubella syndrome. Defects include decreased growth, mental retardation, small head, congenital cataracts or cloudiness of the cornea, deafness, and heart defects.

Taking folic acid supplements may reduce the risk for a group of conditions called neural tube defects by 50 percent.

This parasitic infection can be transmitted to humans through cat feces and can seriously harm an unborn baby. Infection during pregnancy can result in premature delivery, low birth weight, mental and motor retardation, small head, small eyes and blindness, seizures, hydrocephalus, and damage to the liver and blood-forming organs.

Many prescription and over-the-counter medications aren't safe for pregnant women to take. An expectant mother should check with her doctor before taking any medication or supplement.

Amniocentesis usually is done between the 15th and 20th week of pregnancy. Down syndrome is one of the most common genetic chromosomal abnormalities tested for. The number of conditions that can be diagnosed by amniocentesis increases yearly. Amniocentesis can be used to diagnose many chromosomal abnormalities, DNA-gene abnormalities, and enzyme and amino acid abnormalities. In late pregnancy, amniocentesis can even help determine how mature the fetus' lungs are.

Women 35 and older have a greater risk of having a child with chromosomal abnormalities. This is especially true for trisomy 21, also known as Down syndrome.