Prenatal Diagnosis: 11-13+6 Weeks Scan

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Questions and Answers

What is the most important factor in determining pregnancy outcome for twin pregnancies, chorionicity or zygosity?

Chorionicity

What is the mechanism for twin reversed arterial perfusion (TRAP)?

A disruption of normal vascular perfusion and development of the recipient twin due to an umbilical arterial-to-arterial anastomosis with the donor or pump-twin.

Which type of twin pregnancies are associated with a higher risk of miscarriage, perinatal death, preterm delivery, fetal growth restriction and fetal abnormalities?

Monochorionic twin pregnancies.

In which type of twin pregnancies does the risk for chromosomal abnormalities increase with maternal age?

Dizygotic twin pregnancies. Signup and view all the answers

In which type of twin pregnancies is the risk for chromosomal abnormalities the same as in singleton pregnancies?

Monozygotic twins. Signup and view all the answers

How does the risk for chromosomal abnormalities in dichorionic twins compare to that in singleton pregnancies?

The risk for chromosomal abnormalities is twice as high in dichorionic twins compared to singleton pregnancies. Signup and view all the answers

What is the potential for complications associated with fetocide in twin pregnancies?

Complications include spontaneous abortion or severe preterm delivery. Signup and view all the answers

What are the two main options for the management of twin pregnancies with a chromosomal abnormality?

The options include selective fetocide or expectant management. Signup and view all the answers

At what gestational age is fetocide associated with a three-fold increase in risk of spontaneous abortion compared to reduction before 16 weeks?

16 weeks. Signup and view all the answers

What is the advantage of fetal NT screening in dichorionic twins for chromosomal abnormalities?

The presence of a sonographically detectable marker helps ensure the correct identification of the abnormal twin. Signup and view all the answers

How is the risk for trisomy 21 calculated in monochorionic twin pregnancies?

The risk for trisomy 21 is calculated based on maternal age, fetal NT and maternal serum biochemistry. The average risk between the two fetuses is considered to be the risk of the pregnancy as a whole. Signup and view all the answers

What are the three problems with screening by second trimester maternal serum biochemistry in twins?

The three problems include ensuring an acceptable low false positive rate, identifying which fetus may be affected if the pregnancy screens positive, and assessing the increased risk involved in managing discordant pregnancies with chromosomal defects in the second trimester. Signup and view all the answers

What is the advantage of fetal NT screening in dichorionic twin pregnancies for chromosomal abnormalities?

The advantage is that screening for chromosomal abnormalities can be achieved in the first trimester, allowing the possibility of earlier and therefore safer selective fetocide. Signup and view all the answers

Why is the false-positive rate of NT screening in monochorionic twins higher than in dichorionic twins?

The false-positive rate is higher because increased NT is an early manifestation of TTTS. Signup and view all the answers

What is the most effective procedure for providing a reliable karyotype for both fetuses in a twin pregnancy?

Amniocentesis. Signup and view all the answers

Study Notes

The 11-13+6 weeks scan

This scan is used for prenatal diagnosis in the first trimester of pregnancy
It is a method of screening for trisomy 21, in which the excess skin of individuals with Down's syndrome is visualized by ultrasonography as increased nuchal translucency.
The aim is to identify chromosomal defects, such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome)
The scan measures the thickness of the nuchal translucency (NT)
Maternal age is combined with NT thickness to determine and manage the risk of trisomies 21, 18, and 13
It also provides information on the viability of the fetus, accurate pregnancy dating, and identification of major fetal abnormalities and multiple pregnancies
Ultrasound-guided invasive procedures, such as amniocentesis and chorionic villus sampling, are used for more specific diagnosis
A thorough check for other associated features of the chromosomal defect is essential if an abnormality is found
Appropriate training and adherence to a standard technique are crucial for obtaining consistent results
The scan is performed at 11-13+6 weeks' gestation, when the fetal crown-rump length (CRL) is between 45 and 84 mm.

First Trimester Diagnosis of Chromosomal Defects

In 1866 Langdon Down described common characteristics of people with trisomy 21.
Increased nuchal translucency (NT) is a common feature in fetuses with trisomy 21, and 60–70% have absent nasal bone.
Non-invasive prenatal diagnosis aims to isolate and examine fetal cells in maternal circulation.
About 1 in 100–1000 nucleated cells in maternal blood are fetal.
Techniques like magnetic cell sorting (MACS) or fluorescence-activated cell sorting (FACS) can enrich fetal cells.

There are chapters on diagnosis of chromosomal defects, sonographic features of chromosomal defects, and increased nuchal translucency with normal karyotypes.
Other chapters relate to multiple pregnancies and screening for chromosomal defects.
Various methods of screening and diagnosis are discussed, including first trimester screening, non-invasive procedures, and maternal serum biochemistry.
The content includes practical aspects, like training for sonographers, quality assurance processes, patient attitudes and clinical implications.

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Description

This quiz covers the 11-13+6 weeks scan, a vital prenatal diagnostic tool used in the first trimester. It screens for chromosomal defects like trisomy 21, 18, and 13, uses nuchal translucency measurements, and assesses fetal viability. Understanding these aspects helps in managing pregnancy risks and planning further diagnostic procedures.