The ability to assess a pregnant woman’s risk of having a baby with certain abnormalities has grown tremendously in recent decades. This is one of the many reasons why prenatal care is important. The parents may need to make decisions based on the information available.
Knowing about certain conditions beforehand can help the parents prepare to deliver the baby at a hospital capable of providing any special care that may be needed at birth, such as pediatric surgery. It also can give the parents time to learn about and plan for care and support that will be needed later, and to help them prepare emotionally for a child with special needs.
First, it is important to know that not all abnormalities can be detected before birth. A certain amount of risk applies to every situation, including pregnancy and childbirth, despite our best attempts at detection.
There are many possible reasons for abnormalities; among them are genetic defects. Some parents know that certain genetic defects have occurred among family members, and they are aware that there is some risk. In other cases, parents are completely unaware of genetic problems they may have inherited or that may occur spontaneously.
Screening for genetic problems is not performed routinely. Ultrasound exams and blood tests usually are done during pregnancy. However, they do not look for certain “markers” that can indicate genetic defects. The physician should offer genetic screening, or the patient can request it.
There is a difference between screening tests and diagnostic tests. Screening determines the likelihood of an abnormal condition. If the screening shows an increased risk of having an affected baby, a diagnostic test may be performed to confirm whether the abnormality is indeed present. Diagnostic tests to provide more information include a more detailed ultrasound exam: amniocentesis, which requires taking a small of amount of the fluid surrounding the fetus, or chorionic villus sampling, which tests cells taken from the placenta.
Among the genetic abnormalities pregnant women choose to be screened for are Down syndrome, neural tube defects, abdominal wall defects and heart defects.
Down syndrome occurs in one of every 800 births; the risk increases with the mother’s age. In women older than age 45, the risk is one of every 25 births. Down syndrome results in mental retardation and may include heart defects, problems with vision, hearing, digestion and hip dislocation.
A neural tube defect is an incomplete closure of the spine that can result in spina bifida or “split spine,” requiring surgery. In the Midwest, spina bifida occurs in about one in 1,000 births. Heart and abdominal defects also may require surgery. Anencephaly, which is the absence of a large part of the brain and skull, is a fatal condition. The chromosome defect Trisomy 18 is nearly always fatal at birth or within one year.
Screening is done through ultrasound and blood tests at various stages of pregnancy. For instance, during an ultrasound, the technologist can measure the thickness of the skin at the back of the neck. Increased thickness may indicate Down syndrome or a heart defect. A special blood test measures the levels of protein and other factors that also may indicate Down syndrome and other chromosomal problems. In the first trimester of pregnancy, the combined ultrasound and blood test can detect 88 percent of Down syndrome cases. During the second trimester, the tests detect about 90 percent of cases.
Communication with your doctor during pregnancy is important. Be sure to discuss genetics along with any other concerns in preparation for childbirth.