Major developmental or genetic defects occur in more than one in 50 pregnancies. Some result in stillbirths; in others, the baby is born alive. Advances in prenatal diagnosis using the latest obstetrical instruments and equipments have made it possible to identify certain major defects early in pregnancy. For some cases in which the fetus has a serious disorder of the brain and/or spinal cord (e.g., anencephaly or spina bifida), treatment is not possible. The same is true for chromosomal defects, such as the one that produces Down’s syndrome. In other cases, including certain defects of the fetal heart or kidney, or blood disorders such as hemolytic disease of the newborn, early diag­nosis enables treatment to be given even before the baby is born. Early diagnosis also alerts the obstetri­cian and pediatrician to the need for specialized care of the baby immediately after birth.

Ultrasound scanning

Ultrasound scanning is an obstetrical equipment or instrument that is part of the care given to many pregnant women. The scanning technique, which uses the echoes of high-pitched sound waves to con­struct an image of the fetus, is considered harmless and causes no discomfort. The ultrasound image enables the obstetrician to identify multiple pregnan­cies, to measure the size of the fetus (enabling its age to be assessed), and to detect certain physical and developmental defects (such as anencephaly, spina bifida, or some congenital heart abnormalities) early in the pregnancy.

Fetoscopy

Ultrasound provides relatively clear images of a developing fetus, but in some cases it is necessary to examine the fetus directly or to carry out minor procedures on him or her. These procedures can be performed by fetoscopy, in which a fetoscope (a type of endoscope, or fiberoptic viewing tube) is passed into the woman’s uterus through a small inci­sion in her abdomen.

The direct access that fetoscopy gives allows any external fetal abnormalities, such as spinal column defects, facial clefts, or limb defects, to be assessed; it also enables samples of skin or blood to be taken from the fetus for tests. By attaching special instruments to the fetoscope, the physician can perform a variety of procedures, such as transfusing blood into the fetus to treat anemia. The fetoscope also allows some types of disorders (such as obstruction of the urinary tract) to be surgically corrected.

Amniocentesis and chorionic villus sampling

Amniocentesis and chorionic villus sampling can be used to diagnose certain fetal abnormalities early enough in pregnancy for elective abortion to remain a feasible option. However, because these procedures carry a small risk of inducing miscarriage, they are recommended only when there are good medical reasons for doing so, such as when the woman is older than 35 or when there is a family history of a chromosomal abnormality.

Amniocentesis involves removing and testing a sample of the amniotic fluid that surrounds the fetus. The procedure cannot be done before about the 16th to 18th week of pregnancy because there is not enough amniotic fluid before this time.

The amniotic fluid contains fetal cells that can be examined under a microscope, which are cultured to provide chromosomes for a karyotype analysis (a preparation of the chromosomes suitable for identifying chromosomal abnormalities). It may take two to three weeks to culture a chromosome sample using a high-powered microscope, which means that results are not usually available until the 18th to 20th week of pregnancy. The amniotic fluid also contains various chemicals, the levels of which may be measured to test for other, nonchromosomal disorders of the fetus. For example, raised levels of alpha-fetoprotein may indicate a neural tube defect, such as spina bifida.

Chorionic villus sampling is another method of diagnosing fetal abnormalities due to chromosomal defects. Unlike amniocentesis, it is not suitable for detecting nonchromosomal abnormalities. In chorionic villus sampling, a small sample of tissue from the fetal side of the placenta is removed using obstetrical instruments; the cells in the sample (which are genetically identical to those of the fetus) then undergo chromosomal analysis, which can usually be done immediately. Chorionic sampling can be done as early as eight to nine weeks into pregnancy (or later, if necessary). It gives results earlier than amniocentesis and permits a safer elective abortion if this course of action is chosen.