There are two methods used to extract the sample depending on the location of the placenta.  If the placenta is at the front then a needle is passed through the abdomen to collect the sample, otherwise, a catheter is passed through the cervix into the uterus and the sample is taken from there.  In both cases ultrasound is used to locate the placenta and guide the sampling tool, therefore it is necessary that the bladder is full to optimise the scan.

During the procedure, the woman may feel a little discomfort similar to a pap smear.

The risk of miscarriage as a consequence of this procedure are higher than amniocentesis.  The actual rate depends on who is providing the data, some locations quote  between 0.5% to 1% (1 in 200 to 1 in 100) while others quote between 1.5% and 2% (1 in 66 to 1 in 50).  Obviously these are significant risks and need to be discussed in detail between both parents and your medical practitioner or genetic counselor.

Timing: Nowadays, this test is normally taken between the beginning of the 12th week (10 weeks from conception) and end of the 13th week (11 weeks from conception).   When the test was first introduced, it was performed as early as 8 weeks gestation, but several reports of birth defects and higher rates of miscarriage  associated with the procedure has prompted most practitioners to wait until the 10th week of gestation before recommending this procedure.

NOTE:  In several countries, this test is deemed too risky by the authorities and is only offered as an option under very special circumstances (such as a family history of problems or previous chromosomal disorder foetuses). 

The results of the analysis of the biopsy that is taken can be obtained within 1 to 2 weeks and can discover about 200 different disorders with a very high accuracy (99% to 100%) in the diagnosis.

After the procedure, the mother should take lay down rest for at least 1 day and not lift anything for at least a week.  Some very light cramps are normal for the first day, but any prolonged or heavy cramping or fever should be reported.  Some discharge of fluid or blood from the vagina occurs in about a third of cases but should stop within a day or so, if it doesn't then you should report it immediately to your practitioner.  A follow-up ultrasound is normally taken within 2 days of the procedure to ensure that the foetus shows no signs of distress as a result of the procedure.

Introduction

1. The Triple Test - Overview

2. The Triple Test - Detail and Results

3. Amniocentesis

4. CVS

Part 2 - Genetic Disorders