First and Second Trimester Screening

Detailed ultrasonographic examination should be performed at 16–18 weeks’ gestation to evaluate the fetal anatomy and to assess whether the nuchal measurement is in the normal range or is persistently increased. A persistently increased or enlarging nuchal measurement would be associated with a high risk of underlying fetal pathology. Another detailed ultrasonographic examination and fetal echocardiography should be performed at 20–22 weeks’ gestation.

If the increased nuchal translucency measurement in the first trimester resolves over the next few weeks and the fetus continues to appear normal by sonography, which includes fetal echocardiography at 20–22 weeks’ gestation, the most likely outcome of the pregnancy will be a normal baby. However, there remains a small chance of underlying fetal pathology of varying severity that will not be evident by prenatal sonography.

In the patient’s pregnancy, detailed ultrasonographic examination at 16 weeks’ gestation revealed normal-appearing fetal anatomy and fetal growth. Fetal echocardiography at 22 weeks’ gestation revealed Tetralogy of Fallot.

The absence of other fetal abnormalities on ultrasound examination suggests that the heart defect is isolated. However, an underlying syndrome associated with congenital heart disease remains a possibility. Congenital heart defects are commonly seen in the 22q11 microdeletion syndrome (also known as DiGeorge syndrome or velocardiofacial syndrome). This syndrome has variable intrafamilial expression and is often associated with congenital heart disease and other abnormalities including thymic (T cell) immune deficiency, hypoparathyroidism, dysmorphic facies, palatal abnormalities including clefts, learning disabilities, mild mental retardation, seizures, autism, schizophrenia, bipolar disorder, and renal dysplasia. About 15% of individuals with Tetralogy of Fallot also have a chromosome 22q11 microdeletion.

In the setting of congenital heart disease, with or without an increased nuchal translucency measurement in the first trimester, fluorescence in situ hybridization (FISH) analysis using a probe for the chromosome 22q11 microdeletion or array comparative genomic hybridization should be offered.

In this patient’s pregnancy, FISH analysis revealed the presence of the chromosome 22q11 microdeletion. Predictions about the severity of problems associated with the microdeletion syndrome are not possible although some assessment of the presence and severity of T-cell deficiency could be addressed with a fetal blood sample.

Risk of recurrence of the chromosome 22q11 microdeletion syndrome in a future pregnancy depends on whether the microdeletion arose de novo (which occurs in 93% of cases), or whether it was inherited from an affected parent who had unrecognized features of the disorder. Therefore, FISH analysis of parents’ peripheral blood cell chromosomes for the chromosome 22q11 microdeletion is indicated. If one of the parents has the microdeletion, risk of recurrence in a future pregnancy would be 50% although predictions about the severity of expression of the disorder would not be possible. If neither parent has evidence of the microdeletion in their peripheral blood chromosomes, risk of recurrence would be very small, less than 1%, but increased over the general background risk. This small increased risk reflects the possibility of gonadal mosaicism for the microdeletion in one of the parents.

Had fetal echocardiography revealed normal-appearing fetal anatomy, the yield of further molecular testing to investigate a genetic basis of the increased nuchal translucency measurement is small. It is currently not feasible to perform molecular analysis of genes for multiple different disorders because the incidence of each disorder and the detection rate for disease-causing mutations are small. Array comparative genomic hybridization to look for gene deletions and duplications undetectable by routine chromosomal analysis would have a low yield as most of the genetic disorders known to be associated with a first trimester increased nuchal translucency measurement are caused by point mutations. Case reports suggest that Noonan syndrome may have a significant probability when the fetal phenotype in the second trimester includes pleural effusions or hydrops fetalis. Testing for at least some of the associated genes can be performed.

Markedly Elevated Serum AFP Concentration Unexplained by a Fetal Defect on Ultrasound

A 27-year-old woman is referred because of a serum AFP concentration that is 8.7 multiples of the median. A detailed ultrasonographic examination performed at 18 weeks’ gestation reveals normal-appearing fetal anatomy. In the hands of an experienced sonologist and with optimal visualization of the fetus, 90–95% of open neural tube defects and the majority of ventral wall defects should be identifiable by prenatal sonography.

The differential diagnosis of a markedly elevated serum AFP concentration which is unexplained by a fetal defect apparent on ultrasonographic examination includes the following.

1. Placental pathology is the most likely explanation for the markedly elevated serum AFP concentration. It may represent a fetal-maternal bleed at the uterine-placental interface. This is not a rare occurrence and usually is a transient and isolated event.

Placental abnormalities such as chorioangiomas, placental lakes, and inflammatory disease allow increased transudation of fetal proteins into the maternal circulation. Elevated maternal serum AFP levels in the absence of fetal abnormalities are associated with an approximate threefold increased risk of stillbirth, preterm delivery, and intrauterine growth restriction. In association with an elevated hCG concentration determined at the time of quadruple or integrated risk assessment, the increased risk is six- to sevenfold.

Some maternal autoimmune disease states such as systemic lupus erythematosis can be associated with autoantibodies that can lead to decidual vasculopathy and an elevated serum AFP concentration.

2. Fetal disease Some very rare defects of the fetal kidneys and skin can cause leakage of fetal proteins but not be detectable by ultrasonographic examination. One of these is congenital nephrosis, often termed Finnish-type congenital nephrosis, which is a rare autosomal recessive disorder; renal transplantation is often necessary for effective treatment. Disorders of the skin are of variable severity and include cutis aplasia, large hemangiomas, and epidermolysis bullosa. Epidermolysis bullosa is a group of disorders which are characterized by varying degrees of skin fragility and recurrent blister formation. The most severe forms have autosomal recessive inheritance and are often fatal early in life due to severe epithelial blistering of the respiratory, digestive, and genitourinary systems, in addition to skin involvement.

There remains a small chance of an open body defect (i.e., neural tube or ventral wall defect) undetected by ultrasonographic examination. There are also rare reports of families with hereditary persistence of elevated concentrations of AFP.

3. Maternal disease states such as hepatitis, hepatoma, or metastatic or toxic liver disease may also account for the elevated serum AFP concentration.

When the maternal serum AFP concentration is mildly elevated (between 2–3 multiples of the median), the likelihood that the concentration of AFP in amniotic fluid will help to identify a fetal defect is small. Whether to proceed with amniocentesis is debatable in this circumstance and should consider the skills of the sonologist, the risk of amniocentesis, patient preferences, and economic considerations.

However, when the serum AFP is moderately or markedly elevated as illustrated by this case, the risks of underlying fetal pathology undetectable by ultrasonographic examination become greater and amniocentesis for measurement of amniotic fluid AFP should be given serious consideration.

Elevated Amniotic Fluid AFP Concentration

If the amniotic fluid AFP concentration is elevated, this suggests that the fetus is leaking excessive AFP. An alternative explanation would be a placental defect that is allowing leakage of fetal proteins into the amniotic fluid space.

When the amniotic fluid AFP concentration is elevated, further testing should include testing for acetylcholinesterase. Acetylcholinesterase will be present in the setting of neural tube defects and lesions in which neural tissue is exposed. It may not be present in the setting of a ventral wall defect. It would be present in the setting of a severe denuding skin disorder such as epidermolysis bullosa but may not be present for other skin defects. The acetylcholinesterase will be absent in the setting of congenital nephrosis.

Both denuding skin disorders and congenital nephrosis are very rare conditions. It is important to recognize that congenital nephrosis does not occur exclusively among people of Finnish ancestry where the incidence is about 1.2 per 10 000 births. In the United States the incidence of congenital nephrosis is estimated at 1 in 50 000. An estimated 1 out of every 150 000 live births is affected with some type of epidermolysis bullosa. The severe denuding form of autosomal recessive epidermolysis bullosa has an incidence of about 0.5 per million live births.

Providing probability estimates about the likelihood of either of these disorders or other fetal pathology with an unexplained amniotic fluid AFP elevation is inexact. Decisions about further diagnostic testing should take into account the extent of the AFP elevation in amniotic fluid and the presence or absence of acetylcholinesterase.

Congenital nephrosis is genetically heterogeneous; mutations in at least four genes have been associated with congenital nephrosis. Gene sequencing of the NPHS1 gene will identify a high percentage of disease-causing mutations among individuals of Finnish ancestry. However, for individuals of other ancestral backgrounds, the overall sensitivity in detecting disease-causing mutations in the NPHS1 or other congenital nephrosis-associated genes is unknown. In the absence of a family history, gene sequencing may not have utility in the prenatal setting given the low chance of an affected fetus, the unknown mutation detection rate, and the length of time required for molecular analysis. Elevated amniotic fluid albumin levels have been reported in some, but not all, affected pregnancies.

Prenatal diagnosis of epidermolysis bullosa can be accomplished by fetal skin biopsy and examination of the skin by electron microscopy. Fetal skin biopsy is associated with up to 3% risk of miscarriage and an additional few percent risk of extreme prematurity, and is performed in only a few centers nationwide. Sequencing of multiple recessively inherited epidermolysis bullosa-associated genes will find about 95% of mutations when performed in individuals who are affected. In this prenatal scenario, the chance of epidermolysis bullosa is small, and the time frame required for molecular analysis is usually several weeks, which is suboptimal when gestational age is advanced and decisions about pregnancy termination must be made quickly. As developments in gene chip technology and gene sequencing advance, testing for common mutations causing severe epidermolysis bullosa and congenital nephrosis should become faster and less expensive.

Normal Amniotic Fluid AFP Concentration

When the amniotic fluid AFP concentration is normal, a fetal source of the elevated protein concentration seen in maternal serum is very unlikely. This leaves a placental process which is allowing more AFP to transudate into the maternal circulation or a maternal disease state as possible explanations. Sometimes placental abnormalities (e.g., chorioangiomas) are visible on ultrasonographic examination but most often the placenta has a normal appearance.

Maternal disease states such as hepatitis or a hepatic malignancy are rare explanations for an elevated serum AFP concentration during pregnancy. A careful medical history and drug/medication exposure history should be collected. For example, the serum AFP concentration can be elevated with acute liver injury due to acetaminophen toxicity.

When an initial serum AFP determination is ≥5 multiples of the median, a second serum AFP determination should be obtained a few weeks after the initial result to determine whether it continues to rise or is falling. A falling concentration will provide reassurance that a maternal disease state is unlikely. A significant rise in AFP concentration should initiate an evaluation of the mother which would include liver function tests and ultrasonographic imaging of the maternal abdomen. Consultation with other specialists might also be indicated. Amniocentesis can cause transient elevations of serum AFP; therefore, waiting two weeks following amniocentesis for the repeat determination is recommended.

Very Low or Undetectable Concentration of Unconjugated Estriol

The serum uE3 concentration is virtually undetectable in the serum sample of a 26-year-old woman at second trimester (“quadruple”) screening. The serum AFP, hCG, and inhibin A concentrations are unremarkable. A detailed ultrasonographic examination at 18 weeks’ gestation reveals normal-appearing fetal anatomy and growth.

The differential diagnosis of a very low serum uE3 concentration (<0.2 ng/mL or <0.15 multiples of the median) includes the following:

1. Non-pregnant state a woman who is not pregnant will have a very low uE3 concentration.

2. A pregnancy at less advanced date than presumed by the laboratory will have a low uE3 concentration reported because the concentration of unconjugated estriol increases in maternal serum during the second trimester.

3. Intrauterine fetal death is a common explanation for a very low serum uE3 concentration.

4. Fetal pathology can be the explanation.

  • Chromosomal abnormalities such as triploidy, trisomy 18, or Down syndrome can be associated with a very low uE3 concentration.
  • Steroid (placental) sulfatase deficiency associated with X-linked ichthyosis may be present. Steroid sulfatase deficiency is the most common explanation for a very low serum uE3 concentration when the fetus is alive and pregnancy dating is correct. The clinical findings include scaly, dry skin. Steroid sulfatase deficiency is usually of mild or moderate degree and does not have effects on intelligence or longevity. The disorder is commonly mistaken for excessively “dry skin” and is underdiagnosed.

    When the fetus is male and has normal-appearing anatomy as determined by ultrasonographic examination, a very low serum uE3 is associated with a significant risk of X-linked ichthyosis (steroid sulfatase deficiency) whether or not a family history of the disorder is present. The gene is on the short arm of the X chromosome (Xp22.32). A large majority of mutations are gene deletions. A family history of male relatives with excessively dry, scaly, or eczematous skin is frequently elicited.

    Deletions at Xp22 are associated with ichthyosis and, if they extend beyond the steroid sulfatase gene, with other findings such as mental retardation, hypogonadism, chondrodysplasia punctata, and Kallmann syndrome (anosmia/hyposmia, decreased intelligence, abnormal movements and other neurologic findings, and occasional heart and kidney abnormalities). The risk of such an expanded ichthyosis phenotype is small (a few percent).

  • Smith–Lemli–Opitz syndrome is an autosomal recessive disorder of cholesterol biosynthesis with an estimated incidence ranging from 1 in 10 000 to 1 in 40 000. It is the second most common explanation for a low maternal serum uE3 concentration in viable fetuses with correct pregnancy dating. Affected individuals have elevated levels of 7-dehydrocholesterol due to a deficiency of 7-dehydrocholesterol reductase and may have hypocholesterolemia.

    Smith–Lemli–Opitz syndrome has a wide clinical spectrum which, at the severe end, is typically associated with prenatal and postnatal growth retardation, ambiguous genitalia in males, microcephaly, moderate to severe mental retardation, cleft palate, and multiple major and minor malformations. Some affected individuals have only mild symptoms that would not be detectable by prenatal sonography. Intrauterine fetal death may occur for the most severely affected individuals. Maternal serum uE3 concentrations in affected pregnancies range from undetectable to 0.65 multiples of the median, with a median of 0.23 multiples of the median. Using a cut-off of 0.15 multiples of the median (equivalent to a risk cut-off of 1 in 50) will detect about 60% of affected pregnancies; 1 in 300 pregnancies will have a positive screen result at this cut-off (Palomaki et al. 2002).

  • Multiple sulfatase deficiency is a rare and usually fatal autosomal recessive neurodegenerative disorder of infancy and early childhood. It is due to absence of arylsulfatases A, B, and C; arylsulfatase C is steroid sulfatase. Coarsened facial features, bony abnormalities, and ichthyosis are present, plus increased tissue deposition of acid mucopolysaccharides.
  • Adrenal hypoplasia or aplasia is included in the differential diagnosis of a very low serum uE3 concentration. Adrenal hypoplasia/aplasia can be due to a primary adrenal defect; X-linked and autosomal recessive disorders have been reported. Alternatively, hypoplasia of the adrenal glands can be secondary to pituitary or hypothalamic developmental defects, or to structural defects of the central nervous system disrupting pituitary or hypothalamic function. A major structural defect in the brain would be unlikely if intracranial anatomy appears normal by ultrasonographic examination.
  • A number of rare enzyme deficiencies in pathways of steroidogenesis can result in a low serum uE3 concentration, which include: 20,22 desmolase deficiency; 17-alpha hydroxylase deficiency; 3-beta-OH dehydrogenase deficiency; 17,20 desmolase deficiency; blocks in testosterone to estrogen conversion.
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Jul 31, 2016 | Posted by in OBSTETRICS | Comments Off on First and Second Trimester Screening
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