Prenatal diagnosis, also referred to as intrauterine diagnosis or diagnosis before birth, involves the use of various diagnostic techniques, such as imaging, biochemistry, cytogenetics, and molecular biology, to determine whether a fetus is affected by congenital diseases. It provides critical information for subsequent prenatal management, which may include selective termination of pregnancy, intrauterine therapy, or continuation of pregnancy. The primary objective of prenatal diagnosis is to reduce the births of infants with severe, life-limiting, or disabling defects.
Indications
Prenatal diagnosis primarily targets high-risk populations identified through prenatal screening. Common indications for prenatal diagnosis include:
- At least one of the prospective parents having a congenital disease, a family history of genetic disorders, or a consanguineous marriage.
- A history of prior pregnancies with congenital structural abnormalities or genetic disorders.
- Pregnant individuals with recurrent miscarriages, unexplained stillbirths, or neonatal deaths.
- Exposure to substances, radiation, or viral infections known to increase the risk of fetal birth defects.
- Advanced maternal age, typically defined as 35 years or older at the estimated due date.
- High-risk findings for chromosomal abnormalities in prenatal screening.
- Detection of fetal structural abnormalities, growth impairments, or abnormal levels of amniotic fluid, such as polyhydramnios or oligohydramnios, during prenatal ultrasound screening.
- Other situations deemed by healthcare providers to warrant a prenatal diagnosis.
Sampling Techniques
Prenatal diagnostic sampling involves obtaining fetal tissues, cells, or metabolites from the pregnant individual. Sampling methods may be categorized as invasive or non-invasive based on the approach used.
Invasive Sampling Techniques
Invasive methods include amniocentesis, chorionic villus sampling (CVS), percutaneous umbilical cord blood sampling, and fetal tissue biopsy.
Non-Invasive Sampling Techniques
Non-invasive approaches attempt to isolate fetal cells or cell-free fetal DNA from maternal peripheral blood for analysis. Although these methods are promising and hold significant clinical potential, they are still in the stages of clinical research and exploration.
Common Laboratory Diagnostic Techniques
Chromosomal Karyotype Analysis
Chromosomal karyotype analysis can detect numerical abnormalities and structural abnormalities larger than 5–10 Mb in size. Its primary limitations include the lengthy time required for cell culture, significant labor demands, and lower resolution compared to other molecular genetic techniques.
Fluorescence In Situ Hybridization (FISH) and Quantitative Fluorescent PCR (QF-PCR)
FISH uses fluorescently labeled nucleic acid probes to analyze specific chromosomal abnormalities, while QF-PCR leverages fluorescent primers targeting short tandem repeat (STR) sequences to amplify and quantify the sequences. Both techniques provide rapid prenatal diagnosis (typically within 72 hours) for common chromosomal abnormalities involving chromosomes 21, 13, 18, X, and Y. These methods differ from karyotype analysis in that they are limited to detecting targeted chromosomal abnormalities.
Chromosomal Microarray Analysis (CMA)
CMA enables the detection of smaller genetic material gains or losses (10–100 kb) that are not identifiable through conventional karyotype analysis. CMA is recommended when one or more severe structural abnormalities are detected via fetal ultrasound. This method does not require cell culture and provides faster results but is unable to identify low-level mosaics or balanced translocations.
Sanger Sequencing
Sanger sequencing, also known as first-generation sequencing, directly determines the DNA sequence to diagnose known pathogenic mutations. It is considered the "gold standard" of genetic diagnosis and is suitable for detecting point mutations, small deletions, and small insertions. However, its high cost and low throughput have limited its large-scale clinical application.
High-Throughput Sequencing
High-throughput sequencing, also referred to as next-generation sequencing (NGS), can identify genomic variations such as point mutations, small insertions or deletions, and copy number variations. Its short read lengths can pose limitations. Prenatal applications of high-throughput sequencing include targeted gene panels for monogenic diseases, whole exome sequencing (WES), low-coverage whole genome sequencing for chromosomal copy number variation analysis (CNV-Seq), and whole genome sequencing (WGS) for simultaneous diagnosis of monogenic disorders and chromosomal abnormalities.
Detection of Gene Products and Pathogenic Microorganisms
Amniotic fluid, chorionic villi, or fetal blood can be analyzed to detect specific proteins, enzymes, metabolites, or pathogenic microorganisms. These analyses are used to confirm certain fetal metabolic or infectious diseases.
Commonly Used Imaging Techniques
Diagnostic Ultrasound Examination
Diagnostic ultrasound refers to targeted, systematic, and comprehensive imaging examinations performed following the identification of high-risk fetuses during prenatal ultrasound screening. It provides imaging evidence for prenatal counseling and the formulation of treatment plans. However, there are limitations to diagnosing birth defects with ultrasound:
- Structural abnormalities must be sufficiently pronounced to be discernible on ultrasound imaging.
- The timing of the ultrasound examination is critical. Certain conditions, such as spina bifida, holoprosencephaly, dextrocardia, and conjoined twins, can be diagnosed in early pregnancy, while others, such as hydrocephalus, hydronephrosis, and polycystic kidney disease, may only become evident later in pregnancy. Furthermore, some abnormalities may appear during early pregnancy but resolve in subsequent follow-ups.
- Abnormal findings on prenatal ultrasound do not always equate to a definitive diagnosis of congenital fetal diseases. Comprehensive evaluations, including medical history, laboratory diagnostics (such as genetic testing, pathogen testing, and metabolic or enzymatic testing), are often required for a conclusive diagnosis.
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) has become an effective supplementary method to ultrasound in the prenatal diagnosis of fetal structural abnormalities, owing to its high soft tissue contrast, high resolution, multi-planar imaging capability, and large field of view. It is particularly advantageous in cases where complications such as oligohydramnios, excessive maternal intestinal gas, or obesity interfere with ultrasound imaging. Additionally, MRI avoids ionizing radiation, making it a relatively safe modality. No studies have reported harmful effects of a 1.5T magnetic field on fetal development.
Types of Fetal Diseases
Severe Congenital Structural Abnormalities
Structural abnormalities that can be diagnosed prenatally using imaging techniques encompass issues in the fetus and its associated structures. Fetal abnormalities may involve various systems, such as:
- Nervous System: Conditions including anencephaly and spina bifida.
- Cardiovascular System: Conditions such as tetralogy of Fallot and transposition of the great arteries.
- Respiratory System: Conditions including bronchopulmonary sequestration and congenital pulmonary airway malformation (CPAM).
- Placental Abnormalities: Examples include placental hemangiomas and circumvallate placenta.
- Amniotic Fluid Abnormalities: Cases such as polyhydramnios and oligohydramnios.
- Membrane Abnormalities: Examples include amniotic band syndrome.
Chromosomal Disorders
Chromosomal disorders include abnormalities in chromosome number and structure.
- Numerical Abnormalities: This category includes both euploidy (an extra complete set of chromosomes) and aneuploidy (a gain or loss of individual chromosomes).
- Structural Abnormalities: This category includes partial chromosomal deletions, translocations, inversions, and ring chromosomes.
Genomic Disorders
The application of chromosomal microarray analysis (CMA) in prenatal settings has facilitated the detection of microdeletion and microduplication syndromes, such as 22q11.2 deletion syndrome and Cri-du-chat syndrome.
Single-Gene Disorders
Single-gene disorders can be classified based on the affected gene's chromosomal location and mode of inheritance:
- Autosomal Dominant Disorders: Examples include achondroplasia.
- Autosomal Recessive Disorders: Conditions such as phenylketonuria and thalassemia.
- X-Linked Disorders: Examples include Duchenne muscular dystrophy and hemophilia A.
While a few conditions can be treated after birth, effective therapies are unavailable for most single-gene disorders, making prenatal diagnosis particularly critical.
Intrauterine Infections
In cases where maternal infections with pathogens such as Toxoplasma or cytomegalovirus are suspected, amniotic fluid analysis for viral DNA or RNA quantification may assist in determining whether intrauterine fetal infection is present. Amniotic fluid culture remains a reliable method for diagnosing intrauterine bacterial infections. Additional approaches, such as Gram staining of amniotic fluid smears, glucose level measurement, white blood cell counts, and interleukin-6 testing, can support the prenatal diagnosis of chorioamnionitis.