Prenatal Diagnosis: Amniocentesis and CVS
Nothing is as devastating, for a child bearing parent, as the discovery that a long-awaited child has birth defects. In such situations, a reaction of shock, disbelief, and grief accompanied with feeling of guilt and responsibility is experienced. The shattered hopes of parents are usually accompanied with spiritual feelings like – Why me? What wrong did I do to be punished in such a way? Or some people might take things positively and accept things as they are. In earlier days, any defect in the unborn baby was detected only after the baby was born, but ever since new advancement in the field of medical genetics and diagnostic technologies have started assisting reproduction, the growth of this field has become non-stoppable. These advancements have created choice and control in human reproduction by helping to detect any defect in the baby before it is born. Procedures that help to detect birth defect prenatally (before birth) are called as prenatal-diagnosis.
As mentioned above, prenatal diagnosis is the way through which the doctor finds any defect in the yet to be born baby. Mostly prenatal diagnosis helps to detect genetic defects (any change in the normal 46 chromosomes) such as Down syndrome, Turner syndrome etc. The two important and widely used prenatal-diagnoses are Amniocentesis and Chrionic villus sampling (CVS). Some parents are at higher risk of having a baby born with genetic defect, if any one or both parent suffers from any genetic abnormalities. Therefore, they might prefer going for prenatal diagnosis to make sure that they baby is growing healthy. Prenatal diagnosis helps parents to decide about the health care to be provided for the infant. Some diseases can be treated before the baby is born and some right after delivery. But some cannot be treated, in such case the parent need to take decision whether to continue the pregnancy or not.
It is predicted that about 20 – 25% of perinatal (after delivery) deaths occur due to congenital anomalies (birth defects). Prenatal diagnosis helps to
- Plan the coming weeks of pregnancy
- Know the outcome of the pregnancy
- Plan for predicted complications in fetus
- Know the problem in unborn fetus
- Decide whether to continue the pregnancy
- Know problems in future pregnancy
Prenatal diagnosis as a routine
Although, in many healthcare center prenatal diagnosis is offered as a part of routine prenatal care it is not mandatory that every pregnant women should have a prenatal diagnosis. Prenatal diagnosis is mostly considered, in one of the following cases
- In couples who are high risk of having a baby with genetic defect
- If the women is 35 years or above
- Previous child with any birth defects such as Down syndrome or spina bifida
- Abnormal results of screening test done during the first or second trimester. Screening tests include blood test and ultrasound scanning
- If any one of the parent is a known carrier (carries a defective gene but do not show any signs) of cystic fibrosis (a genetic disease affecting the exocrine glands)
Amniocentesis is usually performed during the 15th week of pregnancy of after 15th week. Amniocentesis is aimed at viewing the fetal chromosome, which requires fetal cells. The most common procedure for obtaining fetal cell is by amniocentesis. During amniocentesis, a thin needle is inserted into the uterus through mother’s abdomen under ultrasound guidance and a small amount of amniotic fluid (fluid surrounding the baby) is aspirated out. The amniotic fluid contains fetal cells from fetal skin, lungs, and urinary tract. The aspirated fluid is then sent to laboratory for karyotype analysis (arranging chromosome according to structure and size). During the procedure, the baby will not be hurt and the body will replace the amount of amniotic fluid aspirated. Sometime, during or after the procedure some women may feel cramping, in such case doctor will ask them to rest. Normally, women can return back to normal daily activities the next day itself. Amniocentesis detects genetic defects like Down syndrome, sickle-cell anemia, cystic fibrosis, and Tay-Sachs disease (inherited disorder of lipid metabolism).
Risks involved in amniocentesis
Very rarely amniocentesis carries the risk of miscarriage. The estimated value is 1:200, that is 1 in every 200 amniocentesis may result in miscarriage. Early amniocentesis also increases the risk of foot deformity called as club foot. In such case CVS is considered safer than early amniocentesis. In women, complications like abdominal cramp, amniotic fluid leakage, or spotting from vagina is noted. Fortunately, most of these complications resolve. Infection like fever and chills should be reported to doctor immediately.
Chrionic villus sampling (CVS)
CVS is performed during the 10-13 weeks of pregnancy. CVS provides information similar to the results obtained by amniocentesis. Parents who are willing to go for CVS should have an ultrasound done 2 weeks before CVS is performed to confirm fetal age and fetal viability. CVS involves taking out small sample of the early placenta (chorion frondosum) by inserting a sterile speculum inside the vagina to allow clear visibility of the cervix. Under ultrasound guidance, the catheter is used to remove a small sample of early placenta for genetic analysis. Trans-vaginal CVS is performed when the placenta is low but if the placenta is placed higher the chronic villus sample is removed by inserting a thin needle into the uterus through abdomen. This procedure is similar to amniocentesis. The patient can start doing activities right after the procedure but vigorous activities should be avoided.
Risks involved in CVS
CVS also involves risk of miscarriage, which range from 1:100 to 1:200. If the obtained sample was not sufficient for testing, CVS or amniocentesis should be repeated again. The major risk associated with CVS is limb defect in fetus. However, the risk of limb defect decreases if CVS is performed after 9 weeks of pregnancy as the process of limb development is over.
Amniocentesis Vs CVS
Both the tests have their own pros and cons. There are some situations where amniocentesis is considered better than CVS. Amniocentesis detects neural tube defect, which CVS does not. If the result of other tests like alpha-fetoprotein is abnormal then amniocentesis is preferred. In case if prenatal diagnosis has to be done during the first trimester of pregnancy then CVS is the best option than early amniocentesis.
Prenatal diagnosis is, of course, a major advancement in the field of reproductive genetics. But however, many women who receive confirmed result that the baby has no abnormality cannot rely on the result as no prenatal test can guarantee the birth of a healthy child even after normal results. If the results of prenatal diagnosis are positive then immediate discussion with the physician and counselor is recommended. Discussing the significance of the result with doctor, genetic counselor, and family members is very important to take further decision. Carrying on with the pregnancy or dropping it is up to the parents.