Prenatal screening and diagnosis

SOGC Clinical Practice Guidelines – Prenatal Screening for Fetal Aneuploidy

Obstetrics: Normal and Problem Pregnancies (Gabbe) – Prenatal Genetic Diagnosis

Williams Obstetrics: Prenatal Diagnosis and Fetal Therapy

• Screening: use of specific marker(s) to identify individuals in a population at highest risk for particular disorder
  • Condition being screened for should be severe, and results of screening should affect our management of the pregnancy or the infant’s care after birth
  • An accurate diagnostic test for all individuals who screen positive must be in place, in addition to accessible intervention
  • Markers must be maximally sensitive and specific (i.e. able to identify significant proportion of individuals with condition and minimal number of individuals without the condition)
• Prenatal non-invasive screening tests for Down syndrome, trisomy 18, and open neural tube defects (OTNDs) must be offered to all women for risk stratification prior to consideration of invasive diagnostic methods
• Women >40 years of age (at estimated date of delivery) can opt to undergo invasive diagnostic methods such as CVS and amniocentesis based on age alone, without prior screening (benefits > risks)

First trimester screening for aneuploidy

Screening in the first trimester involves: (i) ultrasound (nuchal translucency) and (ii) a group of serum biochemical markers called the first trimester screen (FTS).

• Nuchal translucency (NT): sonographic visualization of increased thickness of subcutaneous fluid behind the fetal neck above 95th percentile for fetal age; increased size of translucency from 11-14 weeks gestational age associated with increased risk of Down syndrome, as well as other trisomies, Turner syndrome, and congenital cardiac defects.
  Taiwan J Obstet Gynecol. 2010 Jun;49(2):133-8.
  • Mechanism of NTrelates to:
    • Abnormal collagen synthesis: trisomies 13, 18, 21, and Turner syndrome have altered formation of extracellular matrix proteins such as collagen, which results in increased NT.
    • Defects in lymphatics formation: hypothesized to be central defect leading to both NT and cardiac defects. An underlying endothelial differentiation defect causes abnormal lymphatic vessel formation, which in turn leads to accumulation of fluid behind neck. Cardiovascular defects may also stem from this underlying endothelial defect.
• FTS serum biochemical markers: pregnancy-associated plasma protein A (PAPP-A) and free b-hCG
  • Decreased PAPP-A and increased free b-hCG in patients carrying fetuses with Down syndrome
• Consideration of maternal age, nuchal translucency, and serum levels of PAPP-A and b-hCG detects 83% of cases of Down syndrome (with 5% false positive rate)

Second trimester screening for aneuploidy

Quad screen: Maternal serum alpha fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), dimeric inhibin-A (DIA) measured at 15-20 weeks gestation

• Detects 77% fetuses with Down syndrome with 5.2% false positive rate

Ultrasound screening for gross structural anomalies

• Occurs at 18-20 weeks gestational age
• Aneuploidy (see Aneuploidy chapter) commonly associated with significant anatomical anomalies
• “Soft markers” on ultrasound in second trimester confer increased risk of chromosomal abnormalities, including nasal bone hypoplasia, brachycephaly, and clinodactyly

• Open neural tube defects: anencephaly, spina bifida, cephalocele, rare forms of spinal fusion anomalies
  • Risk factors: family history, autosomal recessive syndromes (e.g. Joubert, Roberts syndromes), aneuploidy, maternal hyperglycemia, medications (e.g. valproic acid, carbamazepine)
• Maternal serum AFP at 15-20 weeks > 2.0 or 2.5 multiples of the median (MoM) of unaffected pregnancies associated with neural tube defects
• Specialized sonography (ultrasound) findings:
  • “Lemon sign” (frontal bone scalloping) and “banana sign” (cerebellar bowing with cisterna magna effacement) associated with open spina bifida; seen in second trimester fetuses
  • Visualization of spine in transverse and sagittal orientations typically sufficient to describe location and size of spinal defects
• Amniocentesis should be offered to all patients with positive screen, regardless of ultrasound findings
  • Amniocentesis to analyze AFP and acetylcholinesterase (AchE): detection of AchE suggests open neural tube defect or other structural defects

Summary of first and second trimester screening options

• Ministry of Health and Long-Term Care – Ontario Prenatal Screening (website)
• Williams Obstetrics – Prenatal Diagnosis and Fetal Therapy

In Ontario, different screening modalities are offered before 14 weeks gestation (early prenatal screening) and after 14 weeks gestation (late prenatal screening) for Down syndrome, trisomy 18, and open neural tube defects.

Early Prenatal Screening Modalities (adapted from MOHLTC website)

Late Prenatal Screening Modalities (adapted from MOHLTC website)

Patients in Ontario can elect to pursue integrated prenatal screening or triple/quadruple screening independently (if prenatal screening is not elected to be done until after 14 weeks). In most centers, quadruple screening has replaced triple screening. Stepwise and contingent sequential screenings are not options available in Ontario.

Integrated prenatal screen (nondisclosure)

• Highest detection rate achieved if screening tests conducted in both first and second trimesters – first trimester results not disclosed until second trimester results available
• Nondisclosure approach increases detection rate by 5% in comparison to first trimester screen alone; however ethical concerns regarding delayed notification of first trimester results and thus increased maternal risk with delayed termination options

Stepwise sequential screening (disclosure)

• High risk cohort based on first trimester screening results offered invasive prenatal diagnostic investigations
• Moderate and low risk cohorts all receive second trimester screening

Contingent sequential screening (disclosure)

• Risk stratification based on first trimester screening results – high risk cohort (risk above 1/30) to undergo CVS; low risk cohort (less than 1/1500) does not receive further investigations; moderate risk cohort (risk between 1/1500 and 1/30) to undergo second trimester serum testing
• Most cost effective among 3 variations of screening schedules since approximately 85% of patients will not undergo second trimester screening

SOGC Clinical Practice Guidelines – Canadian Guidelines for Prenatal Diagnosis: Genetic Indications for Prenatal Diagnosis

Obstetrics: Normal and Problem Pregnancies (Gabbe)

Prenatal genetic diagnosis: involves an invasive procedure (amniocentesis, chorionic villus sampling, or fetal blood sampling) to conduct subsequent cell culture and chromosomal evaluation

Indications for offering invasive prenatal genetic diagnosis

• Results from first or second trimester maternal serum screening which suggest aneuploidy (see Prenatal screening above)
• All women with previous abortus, stillbirth, or livebirth with trisomy or other chromosomal abnormality (an exception is Turner syndrome)
• Woman or partner mosaic for chromosomal anomaly or has chromosomal rearrangement
• Parents who are carriers/affected individuals with X-linked disorders
• 2+ relatives with Down syndrome
• Exposure to therapeutic radiation in males: associated with numerical and structural abnormalities in sperm which persist many years after treatment
• Pregnancies conceived by intracytoplasmic sperm injection
• At-risk individuals for microdeletion/microduplication syndromes, such as DiGeorge (22q deletion), Beckwith-Wiedemann (11p duplication), Prader-Willi/Angelman (15q deletion) syndromes
• Abnormal sonographic findings
  • Presence of major fetal anomalies: e.g. multiple congenital anomalies, neural tube defects, cystic hygroma, limb abnormalities, omphalocele, duodenal stenosis/atresia, significant ventriculomegaly, facial abnormalities; or in association with IUGR or variations in amniotic fluid volume
  • Presence of minor fetal anomalies (“soft signs”) during the second trimester associated with chromosomal abnormalities (see Prenatal screening above)