Prenatal Diagnosis: 11-13+6 Weeks Scan

Chorionicity

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.

Monochorionic twin pregnancies.

<p>Dizygotic twin pregnancies.</p> Signup and view all the answers

<p>Monozygotic twins.</p> Signup and view all the answers

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

<p>Complications include spontaneous abortion or severe preterm delivery.</p> Signup and view all the answers

<p>The options include selective fetocide or expectant management.</p> Signup and view all the answers

<p>16 weeks.</p> Signup and view all the answers

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

<p>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.</p> Signup and view all the answers

<p>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.</p> Signup and view all the answers

<p>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.</p> Signup and view all the answers

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

<p>Amniocentesis.</p> Signup and view all the answers

The appearance of a subcutaneous collection of fluid behind the fetal neck in the first trimester of pregnancy.

The optimal gestation for measuring NT is 11 weeks to 13 weeks and 6 days.

The minimum crown-rump length (CRL) for an accurate NT measurement is 45 mm.

The maximum CRL for an accurate NT measurement is 84 mm.

The measurement should only include the fetal head and upper thorax.

The magnification should be large enough so that a 0.1 mm change in the measurement is visible.

The maximum thickness of the subcutaneous translucency between the fetal skin and the cervical spine should be measured.

If the umbilical cord is wrapped around the fetal neck, measure NT above and below the cord, then average the two measurements.

The risk of fetal loss from amniocentesis is about 1% higher compared to ultrasound alone.

Chorionic villus sampling (CVS) performed before 10 weeks is associated with fetal limb abnormalities.

The risk of fetal loss following first-trimester transabdominal CVS is the same as amniocentesis in the second trimester.

The term "screen positive rate" is used interchangeably with "invasive testing rate" in prenatal screening for trisomy 21.

The term "false positive rate" is used interchangeably with "screen positive rate" since most women with a positive screen undergo invasive testing and the majority of fetuses in this group are normal.

Maternal age is the first method of screening for trisomy 21, but it is not as effective as newer methods involving biochemistry and ultrasound.

Maternal serum biochemistry, measuring fetoplacental products in the mother's blood, is more effective than maternal age alone for screening for trisomy 21.

Screening for trisomy 21 using a combination of maternal age and NT measurements at 11-13+6 weeks can identify about 75% of affected fetuses.

Combining maternal age, NT, and maternal serum biochemistry (free b-hCG and PAPP-A) at 11-13+6 weeks improves the detection rate of trisomy 21 to about 85-90%.

The nasal bone is not visible in about 60-70% of fetuses with trisomy 21, but only in about 2% of normal fetuses.

Analyzing fetal cells from maternal blood can be used to assess the risk of chromosomal defects.

The individual risk of chromosomal defects is calculated by multiplying the a priori risk (based on maternal age and gestation) with the likelihood ratios of different screening tests.

The likelihood ratio for a sonographic or biochemical measurement is calculated by dividing the percentage of abnormal fetuses by the percentage of normal fetuses that have that specific measurement.

The risk of many chromosomal defects increases with maternal age, but the risk decreases with gestational age due to the increased likelihood of spontaneous miscarriage.

A woman who has had a previous pregnancy with trisomy 21 has a higher risk of recurrence in subsequent pregnancies compared to the general population.

The risk of trisomies 18 and 13 increases with maternal age and decreases with gestation, with a high rate of miscarriage by 40 weeks.

The risk of Turner syndrome (45,XO) does not change with maternal age, but the prevalence decreases with gestation because of high rates of fetal death.

The risk of other sex chromosome abnormalities (XXX, XXY, XYY) does not change with maternal age, and since the rate of fetal death is not higher than in normal fetuses, the prevalence does not decrease with gestation.

Polyploidy affects a significant proportion of conceptions, but it's highly lethal and rarely seen in live births due to high miscarriage rates.

NT thickness increases with CRL, so it's important to take gestational age into account when assessing if a measurement is above the normal range.

The a priori risk for trisomy 21 is multiplied by the likelihood ratio for NT thickness to calculate the patient-specific risk.

The distribution of NT measurements should be assessed regularly to ensure the quality of the screening program.

The success of a screening program also depends on the training of sonographers and their adherence to standardized measurement techniques.

Large-scale interventional studies have shown that NT screening can identify over 75% of fetuses with trisomy 21 for a 5% false positive rate.

While NT screening preferentially identifies trisomic pregnancies destined to miscarry, the overall impact on the prevalence of trisomy 21 in live births is significant.