Maternal screening – Part 2 – Prenatal Screening For Trisomy 18, Down’s syndrome and Neural Tube Defect (Triple Screening)
- The serum of the patient is taken during the second trimester.
- Ideally around 18th week is the best time.
- Prenatal screening is done to diagnose:
- Neural tube defect
- Trisomy 18
- Down’s syndrome
- This is done for the pregnant ladies to prepare them mentally for abnormal babies.
- This test is advised for the ladies who:
- Has a family history of birth defects.
- ladies 35 years or older age group.
- If there is a history of drugs used during pregnancy which may be harmful to the fetus.
- Diabetic ladies using insulin.
- If there is a history of viral infection during pregnancy.
- If the lady exposed to radiation even on routine X-rays.
- This test may be advised in case of multiple pregnancies.
- Triple screening measure the level of AFP, HCG, and estriol.
- This is a screening test and not confirmatory for fetal abnormalities.
- There may be a false positive test.
- This can be supported by ultrasound.
- For more accuracy advised amniocentesis in positive cases.
Neural tube defect
- Neural tube defect is due to failure of fusion of the neural tube.
- Neural tube defect may show as:
- Spina bifida (open)
- Anencephaly is a serious condition and most babies die within a few hours of the birth.
- The incidence of neural tube defects is one per 1300 pregnancies.
- This is a chromosomal abnormality showing aneuploidy.
- This may show:
- Congenital heart disease.
- Renal abnormality.
- Intellectual abnormality.
- The ears are low-set.
- Fists are clenched.
- The incidence is 1 per 4100 of pregnancies.
- The incidence of trisomy 18 abnormality increases with the age of the mother.
- Baby with trisomy 18 abnormality, 60 % dies in the first month of life.
- While 90 % dies within the first year of life.
- Trisomy 18 usually shows a characteristic pattern that there is:
- Decreased HCG.
- Decreased AFP
- Decreased UE
- About 80 % of Trisomy 18 cases detected by the above screening.
- This syndrome is characterized by trisomy 21 and is common autosomal aneuploidy.
- Down’s syndrome baby shows:
- Retarded growth.
- There is a lack of muscle tone.
- There is an intellectual abnormality.
- The incidence increases with the increasing age of the mother.
- The incidence is 1 per 750 live birth of the baby.
- The triple screen test consists of the estimation of:
- Human chorionic gonadotropin (HCG).
- Alpha-fetoprotein (AFP)
- Unconjugated estriol (UE)
- The sensitivity of this panel to Down’s syndrome is 70 %.
Quad screening test consists of:
- Human chorionic gonadotrophin (HCG).
- Alpha-fetoprotein (AFP).
- Unconjugated Estriol (UE).
- Dimeric inhibin A
- The accuracy of the quad test for Down’s syndrome is 80 %.
- The combined serum test and ultrasonographic measurement of nuchal fold thickness lead to sensitivity to more than 90 %.
Down’s syndrome shows the following pattern:
- Increased HCG
- Decreased AFP.
- Decreased UE.
Neural tube defect shows the following pattern:
- Normal HCG.
- Increased AFP
- Decreased UE.
- Ultrasound will be helpful to evaluate the gestational age, exclude the multiple gestations and can find anatomical abnormalities.
- Amniotic fluid may be obtained by the amniocentesis for the estimation of AFP and acetylcholinesterase.
|Abnormality||HCG||AFP||UE||Dimeric Inhibin A|
|Neural tube defect||normal||increased||decreased|
Conditions associated with increased maternal AFP are:
- Neural tube defect.
- Fetal death.
- Feto-maternal hemorrhage.
- Sacrococcygeal Teratoma.
- Cystic teratoma.
- Cystic hygroma.
- Intestinal obstruction.
- Renal anomalies.
- Turner syndrome.