Prenatal Diagnosis
Prenatal Diagnosis is the diagnosis of disease or condition in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube defects, chromosome abnormalities, genetic diseases and other conditions. Congenital anomalies account for 20 to 25% of perinatal deaths. Prenatal Diagnosis allows parents to plan in advance about the health needs of their baby and also to be mentally prepared.It also helps them to access conditions that may affect future pregnancies.

There are a variety of non-invasive and invasive techniques available for prenatal diagnosis. Each of them can be applied only during specific time periods during the pregnancy for greatest utility. The non-invasive techniques employed for prenatal diagnosis regularly include:

• Ultrasonography
• Fetal blood cells in maternal blood
• Maternal serum alpha-fetoprotein
• Maternal serum beta-HCG
• Maternal serum estriol

Confirmatory diagnostic tests include:

• Amniocentesis
• Chorionic villus sampling

Risk analysis by non-invasive methods:

Ultrasonograph
This is a non-invasive procedure which uses high frequency sound waves to produce visible images from the pattern of the echos made by different tissues and organs , including the baby in the amniotic cavity.The developing embryo can first be visualized at about 6 weeks gestation. Abnormalities in the major internal organs and extremities can be determined between 16 to 20 weeks gestation. This technique helps to determine the size and position of the fetus, the size and position of the placenta and the amount of amniotic fluid. However, subtle abnormalities cannot be detected until in the later stages of pregnancy or even may not be detected at all. A good example of this is Down syndrome (trisomy 21) where the morphologic abnormalities are often not marked


Fetal blood cells in maternal blood
This is a new technique that makes use of the phenomenon of fetal blood cells gaining access to maternal circulation through the placental villi. The fetal cells can be sorted out and analyzed by a variety of techniques like Fluorescence in-situ hybridization (FISH) which identifies particular chromosomes of the fetal cells recovered from maternal blood and diagnose aneuploid conditions such as the trisomies and monosomy X.
Unavailability of fetal blood cells is the prime limitation of this technique. There may not be enough to reliably determine anomalies of the fetal karyotype or assay for other abnormalities.

Maternal serum alpha-fetoprotein (MSAFP)
Albumin and alpha-fetoprotein (AFP) are the two major blood proteins found in the developing fetus. Since adults typically have only albumin in their blood, the MSAFP test can be utilized to determine the levels of AFP from the fetus. Ordinarily, only a small amount of AFP gains access to the amniotic fluid and crosses the placenta to mother's blood. However, when there is a neural tube defect in the fetus, from failure of part of the embryologic neural tube to close, then there is a means for escape of more AFP into the amniotic fluid. Neural tube defects include anencephaly (failure of closure at the cranial end of the neural tube) and spina bifida (failure of closure at the caudal end of the neural tube).

Greater utilization of the test can be achieved with known gestational age. This is due to the fact that the amount of MSAFP increases with gestational age (as the fetus and the amount of AFP produced increase in size). However, the MSAFP can be elevated for a variety of reasons which are not related to fetal neural tube or abdominal wall defects, so this test is not 100% specific. The most common cause for an elevated MSAFP is a wrong estimation of the gestational age of the fetus. Using a combination of MSAFP screening and Ultrasonography, almost all cases of anencephaly and spina bifida can be detected.

Maternal serum beta-HCG
This test is most commonly used as a test for pregnancy. Beginning at about a week following conception and implantation of the developing embryo into the uterus, the trophoblast will produce enough detectable beta-HCG (the beta subunit of human chorionic gonadotropin) to diagnose pregnancy. Thus, by the time the first menstrual period is missed, the beta-HCG will virtually always be elevated enough to provide a positive pregnancy test. The beta-HCG can also be quantified in serum from maternal blood, and this can be useful early in pregnancy when threatened abortion or ectopic pregnancy is suspected, because the amount of beta-HCG will be lower than expected.
In the middle to late second trimester, the beta-HCG can be used in conjunction with the MSAFP to screen for chromosomal abnormalities, and Down syndrome in particular. An elevated beta-HCG coupled with a decreased MSAFP suggests Down syndrome.

Maternal serum estriol
The amount of estriol in maternal serum is dependent upon a viable fetus, a properly functioning placenta, and maternal well-being. The substrate for estriol begins as dehydroepiandrosterone (DHEA) made by the fetal adrenal glands. This is further metabolized in the placenta to estriol. The estriol crosses to the maternal circulation and is excreted by the maternal kidney in urine or by the maternal liver in the bile. The measurement of serial estriol levels in the third trimester will give an indication of general well-being of the fetus. If the estriol level drops, then the fetus is threatened and delivery may be necessary emergently.

Confirmatory Diagnosis by invasive method:

Amniocentesis
This is an invasive procedure in which a needle is passed through the mother's lower abdomen into the amniotic cavity inside the uterus. Amniocentesis is usually preformed between 14 and 20 weeks gestation. Fetal cells which are found within the amniotic fluid can be grown in culture for chromosome analysis, biochemical analysis and molecular biologic analysis. The risks involved in amniocentesis are uncommon. This procedure will result in fetal loss and maternal Rh sensitization. However, contamination of fluid from amniocentesis by maternal cells is highly unlikely.

Chorionic Villus Sampling (CVS)
Chorionic villi are tiny growths found in the placenta. During CVS, a sample of the chorionic villus cells is taken for biopsy. This procedure is done effectively during the first trimester of pregnancy, due to scarce amount of amniotic fluid around the baby. The chorionic villus sample can be collected by putting a thin flexible tube (catheter) through the vagina and cervix into the placenta. The sample can also be collected through a long, thin needle through the belly into the placenta. The cells thus obtained can be cultured for biochemical or molecular biological analysis and also to determine the karyotype of the fetus. CVS has a small but significant rate of morbidity for the fetus. This loss rate is about 0.5 to 1% higher than amniocentesis. Along with the possibility of maternal Rh sensitization there are instances of sampling the maternal blood cells instead of fetal cells.

What is a screening test?

It is very important to remember what a screening test is before getting one performed. This will help alleviate some of the anxiety that can accompany test results. Screening tests do not look only at results from the blood test. They compare a number of different factors (including age, ethnicity, results from blood tests, etc...) and then estimate what a person’s chances are of having an abnormality. These tests DO NOT diagnose a problem; they only signal that further testing should be done. Prenatal screening is combines two screenings:

1. First Trimester Screening

The First Trimester Screen is a new, optional noninvasive evaluation that combines a maternal blood screening test with an ultrasound evaluation of the fetus to identify risk for specific chromosomal abnormalities, including Down’s Syndrome, Trisomy-13 and Trisomy-18. In addition to screening for these abnormalities, a portion of the test (known as the nuchal translucency) can assist in identifying other significant fetal abnormalities, such as cardiac disorders. The screening test does not detect neural tube defects..
The blood screen measures two pregnancy related hormones: hCG and PAPP-A.
The ultrasound evaluation measures nuchal translucency (fluid beneath the skin behind baby’s neck).
This non-invasive procedure combines the results from the blood tests and the ultrasound, along with the mother’s age, to determine risk factors.

How is the First Trimester Screen performed?
The blood screen involves drawing blood from the mother, which takes about 5 to 10 minutes. The blood sample is then sent to the laboratory for testing. The ultrasound is performed by an ultrasound specialist or perinatologist and takes between 20 and 40 minutes. The results are calculated within a week of the testing.

What are the risks and side effects to the mother or baby?
Except for the discomfort of drawing blood, there are no known risks or side effects associated with the First Trimester screen. There is a 5% false positive rate for the test. Parents should be aware of the possibility of receiving abnormal results and then finding, after further testing, that the baby is normal.

When is the First Trimester Screen performed?
The First Trimester Screen is performed between the 11th and 13th week of pregnancy. Because the test is performed so early, it is often used to determine whether a mother should consider undergoing an early (first trimester) diagnostic test, such as chorionic villus sampling, or second trimester amniocentesis.

What does the First Trimester Screen look for?
In babies who are at an increased risk for chromosomal abnormalities, increased fluid is often found in the nuchal translucency. Abnormally high or low hCG and PAPP-A levels are also often found. The first trimester screen combines the results from these three measurements (nuchal translucency, hCG, and PAPP-A) with maternal age risk factors and determines an overall risk factor for chromosomal abnormalities.

What do the First Trimester Screen results mean?
It is important to remember that the First Trimester Screen is a screening test and not a diagnostic test. This test only notes that a mother is at risk of carrying a baby with a genetic disorder. Many women who experience an abnormal test discover later that the test proved false.

You will not be given specific quantitative values for the separate parts of the First Trimester screen. Instead, you will be told whether your results are “normal or abnormal”, and you will be given a risk level by your genetic counselor. The counselor will give you your risk factor for chromosomal abnormalities based on the test results (for example 1/250, 1/1300).
Abnormal test results warrant additional testing for making a diagnosis. Your genetic counselor will discuss the results with you and assist you in deciding about diagnostic tests, such as CVS or amniocentesis. These invasive procedures should be discussed thoroughly with your healthcare provider and between you and your partner. Additional counseling may prove helpful.

What are the reasons for further testing?
The First Trimester Screen is a routine screening that is not an invasive procedure and poses no known risks to the mother or baby. The First Trimester Screen results may warrant additional testing. The reasons to pursue further testing or not vary from person to person and couple to couple. Performing further testing allows you to confirm a diagnosis and then provides you with certain opportunities:
• Pursue potential interventions that may exist (i.e. fetal surgery for spina bifida)
• Begin planning for a child with special needs
• Start addressing anticipated lifestyle changes
• Identify support groups and resources
• Make a decision about carrying the child to term
Some individuals or couples may elect not to pursue testing or additional testing for various reasons:
• They are comfortable with the results no matter what the outcome
• Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
• Some parents choose not to allow any testing that poses any risk of harming the developing baby
• It is important to discuss the risks and benefits of testing thoroughly with your healthcare provider. Your healthcare provider will help you evaluate if the benefits from the results could outweigh any risks from the procedure.

2. Triple Marker/ Second Trimester Screening

The triple screen test is a maternal blood screening test that looks for three specific substances: AFP, hCG, and Estriol.

AFP: alpha-fetoprotein is a protein that is produced by the fetus.
hCG: human chorionic gonadotropin is a hormone produced within the placenta
Estriol: estriol is an estrogen produced by both the fetus and the placenta
It is a non-invasive procedure done through a blood test with little to no known risk to the mother or developing baby.

How is the triple screen test performed?
The triple screen test involves drawing blood from the mother which takes about 5 to 10 minutes. The blood sample is then sent to the laboratory for testing. The results usually take a few days to receive.

What are the risks and side effects to the mother or baby?
Except for the discomfort of drawing blood, there are no known risks or side effects associated with the triple screen test.

When is the triple screen test performed?
The triple screen test is performed between the 15th and 20th week of pregnancy although results obtained in the 16th -18th week are said to be the most accurate.
All pregnant women should be offered the triple screen, but it is recommended for women who:
• Have a family history of birth defects
• Are 35 years or older
• Used possible harmful medications or drugs during pregnancy
• Have diabetes and use insulin
• Had a viral infection during pregnancy
• Have been exposed to high levels of radiation

What does the triple screen test look for?
The triple screen is measuring high and low levels of AFP and abnormal levels of hCG and estriol. The results are combined with the mother's age, weight, ethnicity and gestation of pregnancy in order to assess probabilities of potential genetic disorders.
High levels of AFP may suggest that the developing baby has a neural tube defect such as spina bifida or anencephaly. However, the most common reason for elevated AFP levels is inaccurate dating of the pregnancy.
Low levels of AFP and abnormal levels of hCG and estriol may indicate that the developing baby has Trisomy 21( Down syndrome), Trisomy 18 (Edwards Syndrome) or another type of chromosome abnormality.
Although the primary reason for conducting the test is to screen for genetic disorders, the results of the triple screen can also be used to identify:
• A multiples pregnancy
• Pregnancies that are more or less advanced than thought

What do the triple test results mean?
It is important to remember that the triple test is a screening test and not a diagnostic test. This test only notes that a mother is at a possible risk of carrying a baby with a genetic disorder. The triple screen test is known to have a high percentage of false positive results.
Abnormal test results warrant additional testing for making a diagnosis. A more conservative approach involves performing a second triple screen followed by a high definition ultrasound. If the testing still maintains abnormal results, a more invasive procedure like amniocentesis may be performed.
Invasive testing procedures should be discussed thoroughly with your healthcare provider and between you and your partner. Additional counseling and discussions with a counselor, social worker or minister may prove helpful.

What are the reasons for further testing?
The triple screen is a routine screening that is not an invasive procedure and poses no risks to the mother or baby. The abnormal triple screen results often warrant additional testing. The reasons to pursue further testing or not vary from person to person and couple to couple. Performing further testing allows you to confirm a diagnosis and then provides you with certain opportunities:
• Pursue potential interventions that may exist (i.e. fetal surgery for spina bifida)
• Begin planning for a child with special needs
• Start addressing anticipated lifestyle changes
• Identify support groups and resources
• Make a decision about carrying the child to term
Some individuals or couples may elect not to pursue testing or additional testing for various reasons:
• They are comfortable with the results no matter what the outcome is
• Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
• Some parents choose not to allow any testing that poses any risk of harming the developing baby
It is important to discuss the risks and benefits of testing thoroughly with your healthcare provider. Your healthcare provider will help you evaluate if the benefits from the results could outweigh any risks from the procedure.

Genetic Counselling:

Counselling for the prenatal screening is very important both before and after the test. A details of the family history, medical records and conditions of family members from both the sides need to be provided to the counsellors before doing the test. If the result comes positive for any conditions, proper advice from the counsellor is absolutely necessary. Risk assessment of the disease needs to be interpretated correctly in order to avoid any mental stress/ trauma.

Courtesy:
http://library.med.utah.edu/WebPath/TUTORIAL/PRENATAL/PRENATAL.html