It is possible to detect by biochemical tests of the maternal plasma(or amniotic fluid) certain fetal anomalies.

In amniocentesis a needle is inserted into the uterus through the abdominal wall of the pregnant mother to obtain amniotic fluid.It is usually carried out after about 14 weeks gestation. There is a small risk to the fetus during amniocentasis that can be minimized if combined with ultrasound in order to locate the position of the fetus,placenta and maternal bladder. This procedure must be performed only is there is a strong clinical indication and the diagnosis cannot be made by noninvasive procedures.

Amniocentesis may be misleading if it is contaminated with other fluids such as fetal blood, maternal blood,maternal urine etc. It may also be misleading if it is not fresh or improperly preserved.

Detection of Neural tube defects

The estimation of the concentration of alpha-fetoprotein (AFP) is very useful in the early detection of neural tube defects. AFP is a low molecular weight glycoprotein synthesized mainly in the yolk sac and liver. It is not produced in the normal adult.

AFP diffuses slowly through the capillary membranes and appear in the fetal urine and therefore in the amniotic fluid.It can also be detected in the maternal plasma. Abnormal high concentrations of AFP are found in anencephaly and spina bifida. This is probably due to lekage from exposed neural tube vessels.

In some developed countries all pregnant women have a plasma AFP measured between 16 and 18 weeks of gestation to screen for the presence of a fetus with neural tube defects.

Causes of raised AFP in amniotic include multiple pregnancy and serious fetal abnormalities such as neural tube defects and exomphalos.

It is important to confirm the gestational age by ultrasound ,which also excludes multiple pregnancy as a cause of the raised AFP.

Amniocentesis and estimation of the amniotic fluid AFP should only be performed if the patient is willing to consider termination of pregnancy if the result is positive.It should generally be reserved for women with a family history of neural tube deects or women who have a very high level of plasma AFP.

Amniotic fluid acetylcholinesterase assay is also useful in detecting serious fetal malformations such as neural tube defects and exomphalos but is less widely available than AFP. However the results are less dependent on fetal age than AFP.

Biochemical detection of Down’s syndrome

A low level of maternal plasma AFP (alpha fetoprotein) and a raised HCG level between 16 and 18 weeks of gestation is associated with an increased fetal risk of Down’s syndrome.

Detection of other fetal abnormalities:

Cytogenetic, biochemical or enzymatic assays on cells cultured from the amniotic fluid (or by biopsy of the chorionic villi) may detect some inborn errors of metabolism.

Biochemical assesment of fetomaternal blood group incompatibility

Blood group incompatibility will raise the concentration of bilirubin in the amniotic fluid which can be measured in conjunction with maternal antibody titres.It is expected that the bilirubin concentration decreases during the last half of pregnancy. The optimum time for induction of labour can be determined if it is used in conjunction with tests of fetal maturity. It is also very useful to determine the need for intrauterine transfusion.

Assessment of fetal lung maturity

Biochemical analysis of the amniotic fluid is very useful in the assessment of fetal lung maturity.

Neonatal respiratory distress syndrome (hyaline membrane disease) is a syndrome where immature lungs fails to expand normally at birth, leading to respiratory distress. It is therefore necessary to have evidence of pulmonary maturity before labour is induced.

The synthesis of the surface tension lowering complex (surfactant) begins at about 32 weeks of gestation. The synthesis of surfactants is by cells lining the alveolar walls. 90% of the surfactant is the phospholipid lecithin which contains palmitic acid.

The surfactant (lecithin ) is washed from the alveolar walls into the amniotic fluid, which leads to a steady increase in the concentrations of lecithin and palmitic acid.The concentration of sphingomyelin (another lipid) remains constant in the amniotic fluid. Therefore the concentration of lecithin relative to sphingomyelin can be measures .This is known as the lecithin/sphingomyelin (L/S) ratio.

However these tests are now rarely required as steroids which induce surfactant synthesis are usually given to patients who have premature rupture of membranes/preterm.

References:

Pages 141 to 145.The reproductive system.Chapter 7.Clinical chemistry in diagnosis and treatment.Sixth edition by Philip D. Maine

No related posts.

 

You are welcome to discuss this post/related topics with Dr Shihaan and other experts from around the World in our Pregnancy, Gynaecology and Baby health Forums (www.askdrshihaan.org/forums/).