Fetal Assessment - Human Embryology

Screening test

A test used to identify individuals at increased risk of a condition who may need further testing.

Diagnostic test

A test used to determine more definitively whether a condition is present.

Main purpose of screening tests

Estimate the risk of a condition in an individual.

Main purpose of diagnostic tests

Provide a more definitive determination of whether a condition is present.

Screening test characteristics

Generally non-invasive, may produce false positives or false negatives, predicts risk, and carries low or no fetal risk.

Diagnostic test characteristics

Generally invasive, has fewer false positives or negatives, and may carry some risk to the fetus.

Can a test be both screening and diagnostic?

Yes. Some techniques generally used for screening can also diagnose certain abnormalities in specific circumstances.

Ultrasonography

An imaging technique that uses low-frequency sound waves reflected by tissues of different densities to create images.

Other names for ultrasonography

Sonogram or ultrasound.

Safety of ultrasound

Ultrasound is safe, with no known risks to the mother or fetus.

Ultrasound before delivery

May be used to confirm the position of the baby.

Other uses of ultrasound

Identify some birth defects, assess placental and amniotic fluid abnormalities, examine the heart and heartbeat, and guide needles during amniocentesis or chorionic villus sampling.

Is ultrasound a screening or diagnostic test?

Generally a screening tool, although it can diagnose some severe structural abnormalities.

Examples of abnormalities detectable by ultrasound

Abdominal wall defects, absence of kidneys, significant disruptions to heart chambers, and hypoplastic left heart syndrome.

Hypoplastic left heart syndrome

A condition in which the left ventricle of the heart is very small or absent.

Three-dimensional ultrasound

Often used for imaging at approximately 26–30 weeks of pregnancy.

Maternal serum screening

A prenatal screening method that measures fetal or placental proteins and hormones in the mother's blood.

Safety of maternal serum screening

It has low risk to the mother and fetus but a relatively high false-positive rate.

False-positive rate of maternal serum screening

Approximately 5%, according to the lecture.

Alpha-fetoprotein (AFP)

A protein produced by the fetal liver that increases steadily during gestation.

hCG

A hormone produced by the placenta that can be measured in maternal serum.

Estriol

A hormone produced by the placenta that can be measured in maternal serum screening.

Inhibin-A

A placental hormone measured as part of maternal serum screening.

What can maternal serum screening detect increased risk of?

Neural tube defects and aneuploidies.

Aneuploidy

A chromosome abnormality involving an abnormal number of chromosomes.

What does an elevated AFP level suggest?

It may indicate an increased risk of a neural tube defect, but it does not establish a diagnosis.

MOM in prenatal screening

Multiples of the median; a way to compare a test result with the expected median value for a given stage of pregnancy.

What happens after an abnormal maternal serum screening result?

Follow-up testing may be recommended because screening results estimate risk rather than confirm a diagnosis.

Prenatal cell-free DNA screening

Also called non-invasive prenatal testing (NIPT); analyzes fetal DNA fragments circulating in maternal blood.

Source of fetal cell-free DNA

Apoptosis of cells in the syncytiotrophoblast releases fetal DNA into maternal circulation.

Approximate proportion of maternal plasma cell-free DNA that is fetal

Approximately 10%, according to the lecture.

What happens to fetal cell-free DNA after birth?

It is rapidly cleared from maternal circulation.

How is fetal DNA analyzed in NIPT?

PCR of fetal DNA from a maternal blood sample can be used for genetic screening.

When can NIPT be performed?

Beginning at approximately 10 weeks of pregnancy.

Safety of NIPT

It is minimally invasive and carries low risk to the mother and fetus because it uses a maternal blood sample.

Limitations of NIPT

It does not detect every genetic condition and screens only for selected chromosome abnormalities or specific disorders.

Who may benefit from NIPT?

Higher-risk pregnancies, including pregnancies involving advanced maternal age or parents who carry certain genetic disorders.

Additional information obtainable through NIPT

Selected chromosome abnormalities and sex chromosomes.

False negative

A test result that indicates a condition is absent when the individual actually has it.

False positive

A test result that indicates a condition may be present when the individual does not have it.

Fetal microchimerism

The presence of fetal stem cells in the mother's body after crossing the placenta.

How long can fetal cells persist in the mother?

Some fetal cells can survive for decades after pregnancy.

Where have fetal cells been found in the mother?

They have been found preferentially in damaged tissues, including heart tissue after a heart attack and cesarean-section scars.

Possible roles of fetal microchimerism

Fetal cells may contribute to tissue repair or be involved in cancer or autoimmune disorders; these roles are described as possible rather than definitive.

Can maternal cells enter the fetus?

Yes. Cell transfer occurs in both directions, and maternal cells can be found in offspring in much lower numbers.

Amniocentesis

A procedure in which a needle passes through the mother's abdominal wall to collect amniotic fluid,

Performed as follow-up to other screenings, more accurate than maternal serum screens

Purpose of Amniocentesis

Collect cells for genetic testing, measure metabolic byproducts of fetus, assess fetal anemia or uterine infection, lung maturity before birth.

Chorionic villus sampling (CVS)

A procedure in which a sample of chorionic tissue is collected for genetic testing.

How is chorionic villus sampling performed?

Chorionic tissue is collected using a transcervical catheter or a transabdominal needle.

Which prenatal tests are generally screening tests?

Ultrasound, maternal serum screening, and prenatal cell-free DNA screening (NIPT).

Which prenatal tests are diagnostic genetic procedures?

Amniocentesis and chorionic villus sampling.

Why might an abnormal screening result lead to a diagnostic test?

Screening estimates risk and can produce false positives, so diagnostic testing may be needed to establish whether a condition is present.