Many a couple have had an unsuccessful pregnancy due to abnormalities in the fetus and hence are afraid or skeptical of another pregnancy. Modern obstetrics offers hope to such couples in the form of prenatal diagnosis, which means diagnosing abnormalities in the fetus whilst still in-utero. Thanks to developments in ultrasound and DNA technology, more than 600 abnormalities can be diagnosed prenatally i.e. when the fetus is still inside the uterus and at a relatively early stage of pregnancy. Limiting factors are inadequate information to the couples at risk, and failure or untimely referral to the specialist obstetrician for detection of at-risk pregnancies.

A pregnant woman is advised to undergo a prenatal diagnosis under one or more of the following situations:

• Age 35 or above at time of expected delivery, due to increased incidence of chromosomal abnormalities in the baby delivered to such mothers.
• If either parent has had a birth defect in the spine (spina bifida) or heart (congenital heart disease) or a known chromosomal abnormality, as there is an increased chance that a child will have a related defect.
• If the mother has been exposed to ionizing radiation, therapeutic and illicit drugs which are known to cause abnormalities in the fetus.
• If the mother has had a previous baby with structural or chromosomal abnormalities.
• Genetic disorders in the family such as cystic fibrosis, hemophilia, muscular dystrophy, and hemoglobinopathies.
• If in the present pregnancy there are certain abnormalities detected on ultrasound or blood tests (screening such as the triple test etc) suggestive of an abnormal fetus.
• History of recurrent abortions, because in almost 50% cases there is a chromosomal abnormality.
• If the pregnancy is complicated by polyhydramnios (excess amniotic fluid), fetal growth retardation (early onset), and active maternal infections such as cytomegalovirus, toxoplasmosis, herpes simplex and parvovirus B-19.

Methods of testing are noninvasive i.e. ultrasound, and invasive testing by amniocentesis, chorion villus sampling, fetal blood sampling or cordocentesis and fetal tissue biopsy.

Timetable for Prenatal Diagnostic Procedures

9 - 11 weeks	Transcervical Chorionic villus sampling (CVS)
12 - 14 weeks	Early amniocentesis (EAC)
10 weeks - term	Transabdominal CVS
15 - 17 weeks	Standard amniocentesis (SAC)
18 weeks - term	Cordocentesis

Each of the procedure will be briefly described: Ultrasound This is the most widely used procedure. Gross structural abnormalities can be detected at 14 weeks by a transvaginal scan and at 18-20 weeks by a transabdominal scan. If any major abnormality incompatible to life is detected, the pregnancy can be terminated within the legal period for such procedures. Suspicious findings can, in majority of cases be confirmed by one of the invasive procedures. Several studies have shown no deleterious effect of the usual levels of ultrasound in pregnancy. An ultrasound examination is a prerequisite to all other procedures. It notes fetal viability and number, gestational age, fetal anatomy, placental location apart from position of the uterus and presence of any other uterine pathology. Invasive procedures are performed on an outpatient basis under ultrasound guidance, for precision and avoidance of injury to the fetus or the placenta. Asepsis should be foremost while performing any of the procedures. The woman may resume her normal activities a day or two after the procedure.

Chorion villus sampling Chorionic villi are part of the placenta, which in turn is a part of the conceptus (fetus+placenta). Hence a study of the cells in this tissue will reveal any abnormality in the fetus. This is usually performed at 10-13 weeks gestation either through the transcervical or transabdminal route. A fine cannula attached to a syringe is passed via the vagina and cervix into the uterus. Villi are aspirated by applying negative pressure with the syringe. A biopsy forceps may also be used for this purpose. For the transabdominal approach a thin needle is passed into the uterine cavity traversing the abdominal wall. A syringe is attached to the needle and villi are aspirated by negative pressure. Results based on a direct preparation of spontaneously dividing cells are usually available in 24 to 48 hours, and final results from cultured cells in 10 to 14 days.
Amniocentesis This involves removing amniotic fluid (fluid surrounding the fetus) by means of a fine needle inserted under ultrasound control. The needle is inserted via the abdominal wall into the amniotic cavity, and without causing injury to the fetus. Local anesthesia may or may not be necessary. Amniotic fluid contains cells of fetal origin and hence can be tested for abnormalities in the fetus. The procedure is usually performed between 15- 17 weeks (standard amniocentesis) or even earlier i.e. 12- 14 weeks (early amniocentesis). Only a small amount of fluid is drawn and this will not have any deleterious effect on the fetus. There are no absolute contraindications to the procedure. Results take 2-4 weeks and are accurate in 99% of the cases.
Fetal blood sampling This procedure is also termed as percutaneous ultrasound guided fetal blood sampling (PUFBS) or cord blood sampling (CBS). This is performed at 18 weeks and beyond. Through the abdominal wall a fine needle is inserted into the umbilical vessel (usually the umbilical vein) of the fetus. The fetal blood is drawn for analysis. The small amount of blood drawn, in no way causes distress to the fetus. Results are usually available within 48 to 72 hours. Complications for all the above procedures include pregnancy loss (0.2% - 7.5%), vaginal bleeding and infection. Procedure related complications include leakage of amniotic fluid (1-5%) and fetal trauma for amniocentesis, limb reduction defects (0.2%) for chorion villus biopsy, and bleeding from puncture of fetal vessels (10-40%) in case of fetal blood sampling. Fetal tissue biopsy This is best done at 19-20 weeks gestation. Tissue may be obtained from fetal skin, liver muscle or urine for the diagnosis of some inherited disorders. However the risks of fetal injury are high with this procedure, and hence its use is limited.

Comparison: Success/Failure

PROCEDURE	SUCCESS RATE	FAILURE RATE
AC	> 99%	0.1 - 0.2%
CVS	> 98%	1%
CORDOCENTESIS	94 - 98%	2 %

An informed written consent needs to be taken from the woman prior to performing any of the above tests. The indication for the procedure, along with risks involved, consequences of such testing and likelihood of incorrect results must be explained to the patient in the language she understands. A period of delay from counseling to procedure (thinking time) is appropriate and should ideally be about a week An important aspect, which is being tackled on a war footing, is use of prenatal diagnostic tests for fetal sexing. Sex determination has been banned and those (doctor or/and patient) violating the rules are liable for fines and imprisonment.

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