FIRST TRIMESTER SCREENING OF CHROMOSOMAL & STRUCTURAL ANOMALIES IN THE FETUS

FIRST TRIMESTER SCREENING OF CHROMOSOMAL & STRUCTURAL ANOMALIES IN THE FETUS

What is First Trimester Screen ?

It is a specialized investigation in the first trimester of pregnancy ( 11 wks to 13.5 wks ) where we measure 1 ultrasound & 2 biochemical parameters .

  • B-Hcg & PAPP-A : Biochemical ( PerkinElmer Lab)
  • Nuchal Thickness : Ultrasound.

This marker values are converted into MOM & put into the specific algorithm provided by FMF,UK . In this way the risk of chromosomal anomalies in the fetus are calculated.

Introduction :

  • Though developing countries such as India are still struggling from high maternal & neonatal mortality many more births are occurring now in the hospital.
  • There is increased awareness towards ANC Care . Concepts of skilled prenatal care resulting into healthy babies is gaining momentum.
  • MTP Act 1971 does not allow termination of pregnancy after 20 weeks , whereas we commonly do anomaly scan at 20 weeks.
  • So its need of time to screen the pregnancies for the structural & chromosomal anomalies right in the first trimester .

 A summary of the advantages of the first trimester screen.
  • Earlier detection of an increased risk for carrying a child with Down syndrome or other chromosome problem which allows more time for decision making, including the option of first trimester prenatal diagnosis.
  • Better detection rate (90% versus 60-80%) than the MSAFP (triple/quad screen) for Down syndrome and other chromosome problems.
  • Possible identification of major structural anomalies eg. heart defects associated with abnormal test results through further testing.
  • If the results show an increased risk, several testing options exist.
  • Like the MSAFP (triple/quad screen), this test is noninvasive and safe to the fetus.

Nuchal Translucency

Nuchal translucency (NT) is the sonographic appearance of a collection of fluid under the skin behind the fetal neck in the first trimester of pregnancy.
The term translucency is used, irrespective of whether it is septated or not and whether it is confined to the neck or envelopes the whole fetus.
The incidence of chromosomal and other abnormalities is related to the size, rather than the appearance of NTDuring the second trimester, the translucency usually resolves and, in a few cases, it evolves into either nuchal edema or cystic hygromas with or without generalized hydrops . PATHOPHYSIOLOGY

Increased fetal NT is associated with a heterogeneous group of conditions suggesting that there may not be a single underlying mechanism for the collection of fluid under the skin of the fetal neck Possible mechanisms include:

  • Cardiac defects / dysfunction
  • Venous congestion in the head and neck
  • Altered composition of the extracellular matrix
  • Failure of lymphatic drainage
  • Fetal anemia
  • Fetal hypoproteinemia
  • Fetal infection

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