What is Amniocentesis?
This material is explaining about the condition of amniocentesis and the procedures that take place, describing various organs and the factors that surround the chances of the fetus being delivered.
The degree of haemolysis in the baby cannot be related to the maternal antibody titre and this measurement has no real prognostic significance. With increasing haemolysis, increasing amounts of bilirubin are carried in the fetal blood stream attached to the albumin fraction. Some of the, bpmin-hnund bilirubin is filtered into the amniotic fluid through the mucosa of the upper respiratory tract, and the level of the bilirubin in the amniotic fluid correlates with the degree of haemolysis fairly closely. The quantity of bilirubin in the liquor arrmii can be estimated by spectrophotometry.
In the absence of bilirubin the absorption spectra of amniotic fluid for the range 360 to 630 mu in a spectrophotometer is expressed as a relatively straight line. If albumin-bound bilirubin is present in the liquor, increased absorption is found at a wavelength of 450 mjz and this shows as a characteristic peak. The measurement of the height of the peak above the ‘normal’ optical density at 450 mp gives a fairly close indication of the amount of bilirubin present and of the severity of the haemolytic process.
The condition of the fetus in utero can therefore be assessed by estimating the bilirubin content of the liquor obtained at amniocentesis. Two to 5 ml. of liquor amnii are removed using a lumbar puncture needle which is inserted into the uterus just below the mother’s umbilicus and on the side opposite to that of the fetal spine. The colour of the liquor is inspected and the specimen is subjected to spectrophotometry. When possible the position of the placenta should first be determined using the isotope method.
If the patient develops antibodies during the pregnancy, in a titre of more than 1 in 8, amniocentesis should be performed at discovery provided the pregnancy has advanced to 28 weeks. If there is a history of previous stillbirths, or if antibodies have been present from early pregnancy the first tap is made at the 24th week or even earlier.
The bilirubin peak is entered on to a chart devised by Liley in New Zealand. If the level lies in the high zone, the baby will certainly be severely affected and may die in utero. If the pregnancy has advanced to 35 weeks, labour should be induced; and before that date intra-uterine fetal transfusion offers a 30 % salvage rate, unless the fetus is hydropic when the transfusion is of no benefit.
If the peak lies in the midzone, a further amniocentesis is performed 2 to 3 weeks later and the height of the peak is noted, treatment being planned on this result. Further taps may be required, as the object is to maintain the pregnancy to the 36th week when induction should be performed. If the peak lies in the low zone, a further tap is made at the 34th week. If this tap shows that the peak is now in the midzone, treatment is for that condition, but if the peak remains in the low zone pregnancy may continue to 38 weeks, or to term.
When the initial tap gives a reading in the high zone fetal survival is unlikely, even with fetal transfusions, and only 20 % of such babies survive. A reading in the midzone is associated with a fetal survival rate of 80 % and intra-uterine fetal transfusions are not indicated. Readings remaining in the lower zone are associated with a fetal survival rate of 95%.
Source: ”Diagnostic Tests – Amniocentesis“. Harvard Medical School.
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