Approximately 4 babies in every 100 have birth defects that can be recognised at the time of birth. By the end of the first year of postnatal development, that proportion doubles to about 8% as previously ‘hidden’ problems (for example heart defects or deafness) are recognised. The precise figures differ from one population to another, and there are regional variations even within one population.
A birth defect is an abnormality of structure, function or metabolism (body chemistry) present at birth (whether or not recognised at that time) that adversely affects the individual’s daily life.
The incidence of a birth defect is the proportion of embryos and fetuses developing the defect. However, not all affected embryos and fetuses survive until the time of birth. The proportion of newborn babies with a particular defect is referred to as the prevalence of that defect.
causes of birth defects - aetiology
Both genetic and environmental factors can cause birth defects, either separately or in combination. However, the causes of approximately half of all birth defects are currently unknown.
The human genome contains approximately 19,000 to 20,000 genes, and a fault in one or more of these may result in a birth defect. A baby receives half its genes from each parent. If it receives a faulty gene from one parent and then develops a birth defect as a consequence, that is called dominant inheritance. Examples include achondroplasia (a form of dwarfism) and Marfan syndrome (a connective tissue disease). Some birth defects develop only if the baby receives two copies of the faulty gene, one from each parent. This is called recessive inheritance. An example is cystic fibrosis (a disorder of the lungs and digestive system caused by a change in a chloride ion channel that is involved in the formation of sweat, digestive juices, and mucus). Sometimes, a faulty gene is carried by the X-chromosome. Since the cells of males contain only one copy of the X-chromosome, the faulty gene will be expressed and may result in a disorder such as haemophilia (a blood-clotting disorder) or Duchenne muscular dystrophy (progressive muscle weakness). The cells of females contain two copies of the X-chromosome, so the presence of a faulty gene on one can be compensated for by a normal copy on the other, and the faulty gene is less likely to be expressed.
Abnormalities in the number or structure of chromosomes can result in birth defects. For example, a baby may be born with too many or too few chromosomes, or with one or more chromosomes that are broken or rearranged. If an extra copy of chromosome 21 is present in each cell (trisomy 21), the baby develops the features of Down’s syndrome with characteristic facial features, a possible heart defect, and a degree of mental disability. Babies born with trisomy 18 or 13 have multiple birth defects and usually die in the first months of life.
Missing or extra sex chromosomes (X or Y) may result in altered sexual development and possibly infertility, growth abnormalities, and behavioural problems.
Birth defects also may result from a wide range of environmental factors such as drugs and other harmful chemicals, infections, nutritional imbalances, radiation, and maternal ill-health.
Many birth defects appear to be caused by a combination of one or more genes and environmental factors. This is called multifactorial inheritance. Examples include cleft lip/palate, talipes (clubfoot), and some heart defects.
Heart defects are the most common type of structural birth defect, affecting approximately one baby in 125. Advances in surgery have improved the outlook for affected babies, but cardiovascular defects remain remain a leading cause of infant death.
The neural tube defect called spina bifida affects approximately one in 2,000 babies. There are different degrees of severity, but affected babies may suffer paralysis and bladder and bowel problems. Both genetic and nutritional factors appear to play a role.
About one baby in 135 has a structural defect involving the genitals or urinary tract. These vary greatly in severity, ranging from abnormal placement of the urethral opening beneath the penis (hypospadias) to absence of both kidneys. The cause of hypospadias, which is surgically correctable, is unknown. Babies who lack both kidneys die in the first hours or days of life.
Metabolic disorders affect one in 3,500 babies. These disorders are not visible, but can be harmful or even fatal. Most are recessive genetic diseases. Mostly, these diseases result from the inability of cells to produce an enzyme (protein) needed to change certain chemicals into others, or to carry substances from one place to another. Phenylketonuria (PKU) is a metabolic disorder in which affected babies cannot process a particular amino acid, with the result that it accumulates in the blood and causes brain damage. PKU is routinely detected with newborn screening tests, so affected babies can be placed on a special diet that prevents mental retardation.
If a pregnant woman is infected with rubella (German Measles) in the first trimester, her baby has a one-in-four chance of being born with one or more features of congenital rubella syndrome (deafness, mental retardation, heart defects, blindness). Widespread vaccination has lead to a reduction in the prevalence of this condition.
The most common congenital viral infection is cytomegalovirus (CMV). About 1 percent of all newborn babies in this country are infected, although only about 10 percent of them experience serious consequences such as mental retardation and loss of vision and hearing. Pregnant women generally acquire CMV from young children, who usually have few or no symptoms.
Sexually transmitted infections in the mother can also endanger the fetus and newborn. About one baby in 2,000 is affected.
Other causes of birth defects include fetal alcohol syndrome, which affects one baby in 1,000. This pattern of mental and physical birth defects is common in babies of mothers who drink heavily during pregnancy. Even moderate or light drinking during pregnancy can pose a risk to the baby.
Rhesus (Rh) disease of the newborn is caused by an incompatibility between the blood of the mother and her fetus. It can result in jaundice (yellowing of the skin), anaemia, brain damage and death. Rh disease can usually be prevented by giving an Rh-negative woman an injection of immunoglobulin at 28 weeks of pregnancy and after the delivery of an Rh-positive baby.
prevention of birth defects
While the causes of many birth defects are not yet known, there are a number of steps a woman can take to reduce her risk of having a baby with a birth defect. One important step is to ensure that she is in good health both before and during pregnancy. Testing for some genetic risk factors is now possible. Couples who have had a baby with a birth defect, or who have a family history of birth defects, should consider consulting a genetic counsellor. These health professionals help families understand what is known about the causes of a birth defect, and the chances of the birth defect recurring in another pregnancy. Genetic counsellors also can provide referrals to medical experts as well as to appropriate support groups.
Pre-pregnancy preparation is especially crucial for women with medical problems like diabetes, high blood pressure, PKU, and epilepsy, which can affect pregnancy. Women with poorly controlled diabetes are several times more likely than women without diabetes to have a baby with a serious birth defect. However, if their blood sugar levels are well controlled starting before pregnancy, they are almost as likely to have a healthy baby as women without diabetes. Vaccinations against rubella and chickenpox may be appropriate for some mothers-to-be.
Ideally, women who might become pregnant should take a supplement containing 0.4mg of the B-vitamin folic acid. Studies show that taking this vitamin prior to and in the early weeks of pregnancy reduces the risk of having a baby with neural tube defects.
A woman who is pregnant or planning pregnancy should avoid alcohol, smoking, and recreational drugs — these can cause birth defects and other pregnancy complications. She should not take any medication — prescription, over-the-counter, or herbal — without first checking with her health care provider.
Some birth defects can be diagnosed before birth, using one or more prenatal tests including ultrasound, amniocentesis and chorionic villus sampling (CVS). Ultrasound can help diagnose structural birth defects, such as spina bifida, heart and urinary tract defects. Amniocentesis and CVS are used to diagnose chromosomal abnormalities, such as Down’s syndrome. They also can detect, or rule out, numerous genetic birth defects that may be suspected because of family history or ethnic background.
A small percentage of couples will learn through prenatal diagnosis that their baby has a birth defect. While this news can be devastating, prenatal diagnosis sometimes can improve the outlook for the baby. Advances in prenatal therapy now make it possible to treat some birth defects before birth. For example, biotin dependence and methylmalonic acidemia — two life-threatening inherited disorders of body chemistry — have been diagnosed by amniocentesis and treated in the womb, resulting in the births of healthy babies.
Prenatal surgery has been carried out on babies with spina bifida, urinary-tract blockages, rare tumours of the lung, and congenital diaphragmatic hernia. Prenatal blood transfusions have saved numerous babies with severe Rh disease, and heart medications given to the pregnant woman have saved babies with serious heart rhythm disturbances.