antenatal screening and diagnosis

Question 1

what maternal blood screening is done

Answer 1

• Haemoglobinopathy: Thalassaemia, Sickle cell disease
• VDRL screening
• HIV, Hepatitis B
• Maternal Rhesus antibody
• Combined first trimester serum screening for Trisomy 21 downs, 18 edward and 13 patau

Question 2

when is screening done

Answer 2

maternal age

• first trimester combined screening, fetal NT

If they miss this then they can only do serum

integrated screening

20 week anomaly scan - useful screening tool

Question 3

how many minimal scans are provided for pregnant women

Answer 3

2 scans

1st - dating scan, NT

2nd - detailed anatomy scan between 18-21 weeks

Question 4

other reasons for having additional scans

Answer 4

Response to clinical concerns /findings-small for dates, polyhydramnios, oligohydramnios, reduced Fetal movements

response to raised maternal AFP

referred because they are small 3cm above or below

Question 5

Why is first timester US used

Answer 5

• Viability
• Accurate dating - C sections
• twin determination and chorionicity - confirm it is intrauterine
• detection of fetal abnormalities-anencephaly (absent cranial bone) large anterior abdominal wall defects (gastrachiasis) cystic hygroma
• measurement of nuchal translucency

Question 6

what is nuchal translucency

Answer 6

yellow fluid behind neck in bw image measures the space

Question 7

purpose of a detailed US scan 18-21 weeks

Answer 7

Viability
Measurements (growth)
Liquor volume - around baby 
Fetal anatomy
Placental location
Assessment of normal variants for aneuploidy and fetal growth

Question 8

normal variant screening in pregnancy (soft markers)

Answer 8

• nuchal fold >6 mm
• Ventriculomegaly>10mm
• Echogenic bowel
• Renal pelvic dilatation
• Biometry <5th centile on national charts

Question 9

what should not be reported from a detailed scan

Answer 9

Two vessel cord
Enlarged cisterna magna
choroid plexus cysts
echogenic focus in heart

Question 10

name invasive procedures done in pregnant women

Answer 10

Amniocentesis
Chorion villus sampling

Fetocide
Aspiration of fluid filled fetal cavities
Amnioreduction/amniodrainage

Question 11

indications for amniocentesis/chorion villous sampling

Answer 11

1) Assessment of fetal karyotype- fetal chromosomes

maternal age, high risk for aneuploidy screening, abnormal ultrasound findings, parental translocation, maternal request

2) Molecular genetic testing –
autosomal recessive/ dominant conditions

3) Virology screen
To assess for fetal infections eg CMV

Question 12

what is amniocentesis

Answer 12

getting fluid from around the baby

Ideally after 15 weeks of gestation - if done earlier miscarriage rate may increase

Should be performed under direct Ultrasound guidance
15-20 ml aspirated using 22 G needle

Culture of amniocytes, harvesting and banding; Microarrays, Rapid testing ( FISH/PCR)

Question 13

risks of amniocentesis

Answer 13

Risk of miscarriage 0.5-1%

preterm delivery

chronic liquor leak

Question 14

what is chorion villous sampling

Answer 14

• Ideally after 10 weeks
• sample from placental tissue
• Ultrasound guided with continuous visualisation of needle tip
• Transabdominal/transcervical
  Sample to cytogenetic/molecular genetics

Question 15

risks of chorion villous sampling

Answer 15

Risk of miscarriage 1%
Risk of culture failure 1%
mosaicism 1%

Question 16

what is fetocide

Answer 16

done by fetal medicine specialists

Detection of serious fetal abnormality and aneuploidy often delayed until the anomaly scan 20-22 weeks’ gestation

Distressing for parents to faced with the delivery of their baby with signs of life

Induction of labour with prostaglandins preceded by fetocide after 22 weeks of gestation in many countries.

lidocaine

Question 17

recent advances to get chromosomal problems

Answer 17

Use of molecular cytogenetics-FISH ( Fluorescence in situ hybridisation) visualisation of specific DNA sequences

Use of PCR-for aneuploidy, infections

Question 18

indications for preimplant genetic diagnosis

Answer 18

Known balanced parental translocation

Increased maternal age

Familial history of X-linked recessive disorders

Question 19

advantages of non invasive prenatal diagnosis

Answer 19

Less invasive diagnostic techniques that probe the fetal genome through either isolation and
characterisation of DNA from fetal cells identified in the maternal circulation or analysis of free fetal DNA in maternal plasma.

reduces risk of miscarriage

Circulating fetal nucleated red blood cells, mesenchymal stem cells, and trophoblast have all been used for various prenatal diagnostic tests.

Question 20

what is free fetal DNA

Answer 20

progressively increases during pregnancy, has been estimated to account for approximately 3% to 6% of total DNA in maternal plasma, smaller amounts in serum.
Estimates of the number of fetal cells in the maternal circulation vary depending on the stage of gestation and the method used for analysis.

It is rapidly cleared from the maternal circulation and is undetectable within two hours of delivery.

Question 21

3 areas where non invasive prenatal diagnosis has been used (NIPT)

Answer 21

Prediction of fetal rhesus phenotype from maternal plasma

Prediction of fetal sex from maternal plasma

Detection of fetal aneuploidy

Question 22

Prenatal diagnosis in pregnancy

Answer 22

Determination of chorionicityfetal gender, number of placentas and characteristics of the membrane between the two amniotic sacs

Nuchal translucency measurement

Fetal abnormality

Question 23

how is chorionicity is determined

Answer 23

number of placentas and characteristics of the membrane between the two amniotic sacs

Question 24

characteristic sign of dichrorionic damniottic twins

Answer 24

lambda sign at 10-14 weeks of gestation