Prenatal Diagnosis and Counseling

Principles of Prenatal Screening and Diagnosis

Screening is the systematic application of a test to identify individuals at high risk for an asymptomatic, well-defined serious medical condition with an established incidence, for which identification would lead to prevention and/or treatment. The screening test should be cost effective, simple and safe, readily available and accessible, and should have a well-defined performance. An accurate diagnostic test should be available to confirm or refute the screen positive result. In prenatal screening programs, there should be timely transfer of test results, counseling, respect for the ethical and cultural values and decisions of patients, and full discussion of all options if the suspected condition is confirmed.

Diagnosis of a suspected condition in an at-risk fetus requires a known diagnosis for which the condition is associated with a detectable abnormality either within the fetus and/or the fetal tissues. This could be in the form of a structural abnormality readily seen on ultrasound (that appears at the appropriate developmental stage of the system affected), a cytogenetic abnormality, a biochemical abnormality in the amniotic fluid or in cultured amniocytes, a genomic duplication or deficiency identified on microarray analysis, or a known genetic mutation associated with the condition. For example, a mother with autosomal dominant achondroplasia has a 50% chance of having an affected child. Knowing that in an affected fetus the long bones do not demonstrate growth deceleration until after 24 weeks’ gestation, a normal growth ultrasound at 20 weeks’ gestation would not be reassuring, but a normal growth ultrasound at 30 weeks’ gestation would essentially exclude the diagnosis in her fetus. Conversely, if she had a known mutation for achondroplasia, genetic testing of chorionic villi retrieved at 13 weeks’ gestation or amniocytes retrieved by amniocentesis at 16 weeks would inform the status of the fetus before the condition is visible on prenatal imaging; or, preimplantation testing for the mutation in embryos with transfer of an unaffected embryo would eliminate the risk of an affected fetus.

Midtrimester Genetic Amniocentesis

The term amniocentesis refers to the procedure of removing amniotic fluid under ultrasound guidance from the uterus and is performed for many reasons. For prenatal diagnosis, it is usually performed between 15 and 20 weeks’ gestation. The amniotic fluid contains desquamated cells from fetal skin, bladder, and the gastrointestinal tract, which serve as sources for cytogenetic and enzymatic/biochemical studies. DNA can also be extracted from these cultured amniocytes for genomic studies. Proteins such as alpha fetoprotein (AFP) and acetylcholinesterase are measured to confirm an open NTD suspected on ultrasound.

The benefits of second trimester amniocentesis include its large international clinical experience of over 40 years; the standardization of culture and cytogenomic techniques which decrease the culture failure rate to 0.1%; and its diagnostic accuracy, broad availability, and relative safety. Based predominantly on data obtained in the 1980s and 1990s when second trimester amniocentesis was widely performed, the incidence for minor complications such as cramping and leakage of fluid immediately after the procedure was collectively about 1%, while the incidence of significant complications such as chorioamnionitis and/or miscarriage was 0.25% to 0.5%.

The relative safety of midtrimester amniocentesis when completed by experienced providers has been confirmed by many studies. The multicenter First Trimester and Second Trimester Evaluation Risk (FASTER) trial in 2004 observed a procedural loss rate after second trimester amniocentesis (and before 24 weeks’ gestation) of 0.06% or 1/1600. Prior studies reporting high pregnancy loss rate likely reflect the nuances that contribute to the safety of any procedure that requires technical expertise. Ultrasound guidance, use of a smaller 22-gauge needle, and a large volume of patients at a referral institution allowed practitioners to maintain their technical skills. Each institution should calculate its own complication rate. In 2008, Odibo and colleagues reported on a single center’s 16-year experience and identified a fetal loss rate of 0.13% or 1/769. A 2015 meta-analysis of miscarriage after amniocentesis in more than 42,000 women who had the procedure—compared with 138,000 who did not—estimated the procedural loss rate to be approximately 0.11% or 1/900. These data support that midtrimester amniocentesis is safe when the procedure is performed by experienced providers in large volume referral centers. Thus the current estimated procedural risk discussed with patients is approximately 0.1% to 0.3%. To date, even with maternal serum screening and cfDNA as options for screening, midtrimester amniocentesis remains the procedure indicated for prenatal diagnosis of fetuses with ultrasound anomalies and/or screening results suspicious for fetal cytogenomic abnormalities.

Early amniocentesis , performed between 11 and 14 weeks’ gestation, was briefly explored in the late 1990s. The only large prospective study, the Canadian Early and Mid-Trimester Amniocentesis Trial, randomized 4334 women to early amniocentesis versus midtrimester amniocentesis and observed a higher pregnancy loss rate, more rupture of membranes, more culture failures, and greater procedural difficulty in the early amniocentesis group. An unanticipated observation was a 1.3% incidence of club feet when early amniocentesis was performed between 11 and 13 weeks’ gestation, compared with 0.1% after midtrimester amniocentesis.

The timing of second trimester amniocentesis is based on a number of factors. Midtrimester amniocentesis is usually performed after 15 to 16 weeks of pregnancy because the uterus prior to this gestational age is still within the maternal pelvis and not easily accessible ( Fig. 26.1 ). At midtrimester, the volume of amniotic fluid is about 150 to 300 mL, with the fetal kidneys beginning to produce urine ( Fig. 26.2 ). There is uncertainty about the rate of fetal urine production in early pregnancy, but at 25 weeks’ gestation, the fetal urine output is estimated to be about 110 mL/kg/24 h. For fetal karyotyping and genomic studies, approximately 20 to 40 ml of amniotic fluid is necessary to obtain an adequate number of amniocytes for culture in order to provide results with appropriate accuracy and confidence. While midtrimester amniocentesis has not been associated with permanent structural or functional consequences to the exposed fetuses, the association of early amniocentesis with a 10-fold increase in club feet demonstrates the importance of an adequate amount of amniotic fluid at this developmental stage for normal orthopedic development of the fetus. Midtrimester amniocentesis performed later, at 18 to 22 weeks’ gestation, is technically easier, and there is less concern for removing 40 ml of amniotic fluid. However, cases requiring complex molecular testing may involve weeks of analysis, thus extending completion of testing late in pregnancy and potentially limiting management options.

Chorionic Villus Sampling

Chorionic villus sampling (CVS) involves the aspiration of the chorion frondosum either transabdominally or transcervically between 10 and 13 weeks’ gestation ( Fig. 26.3 ). Trophoblasts and mesenchymal core cells of the chorionic villi provide actively growing cells for karyotype and genomic analyses and biochemical/enzymatic studies. In contrast to second trimester amniocentesis, mosaicism (the finding of two or more cell lines with a different chromosome constitution—usually trisomy) occurs in about 1% to 2% of cases. Because the cell types represent both extraembryonic and embryonic tissue, resolution of a mosaic CVS result requires identification of the source of the cytogenomic abnormality by completing an amniocentesis and, in some cases, fetal blood sampling for further clarification. As a result of CVS, the developmental processes of confined placental mosaicism, trisomic rescue , and uniparental disomy were discovered. Depending on the placental cell lineage from which the chromosome abnormality was derived, confined placental mosaicism could result in (1) generalized mosaicism affecting both the placenta and fetus; (2) mosaicism in the placenta only and a diploid/chromosomally normal fetus; (3) chromosome abnormality confined to the placenta with a chromosomally normal fetus; or (4) chromosomally normal placenta and a mosaic fetus. Trisomic rescue refers to the process by which the zygote began as a trisomic conceptus and, through postzygotic loss of the extra chromosome, became diploid while the placenta remained mosaic or completely abnormal. The clinical consequence of trisomic rescue is chromosome dependent because some genes require the presence of both maternal and paternal copies to express a normal phenotype. In the case of chromosome 15, for example, if CVS demonstrated mosaicism for trisomy 15 and genetic amniocentesis demonstrated that the fetus was diploid for chromosome 15, studies to confirm that biparental inheritance must be completed to predict a normal fetal phenotype. However, if the fetus contained two maternal copies of chromosome 15, it would be predicted to have Prader–Willi syndrome, or Angelman syndrome if it had two paternal copies of chromosome 15.

CVS is associated with a pregnancy loss rate of about 1/500. It requires operator expertise and continuous ultrasound guidance. Maternal cell contamination studies are completed to discriminate between female fetal results and contamination from maternal cells. CVS allows for early diagnosis at a time when the privacy of the pregnancy can still be maintained and should be considered if complex diagnostic strategies (requiring time) are anticipated. Because CVS is dependent on operator expertise within a small gestational age window, it is not readily accessible to all patients.

Characteristics of midtrimester amniocentesis and CVS are compared in Table 26.1 .