Prenatal screening is performed during pregnancy by specific investigations.
Indications for prenatal diagnosis:
- Maternal age over 35 years;
- A child with chromosomal abnormality in history;
- One parent carrier of genetic structural abnormalities;
- Mother carrying a recessive X-linked disease;
- Monogenic diseases in history;
- Parents are carriers of recessive genes, which can be detected by prenatal diagnosis;
- Family history of neural tube defect;
- Screening tests at risk;
- Abnormal fetal ultrasound;
- After in vitro fertilization procedures or intracytoplasmic injection of spermatozoon in egg, which can predispose to chromosomal abnormalities.
The purpose of prenatal diagnosis is to detect birth defects with chromosomal, monogenic, genomic and multifactorial etilogy.
In practice, prenatal screening is used to determine biochemical markers, ultrasound measurements and signs which can highlight genetic abnormalities.
Methods Of Prenatal Diagnosis
- Non-invasive: ultrasound, Doppler ultrasound, biochemical screening, analysis of fetal cells from maternal blood.
- Invasive: chorionic villi biopsy, amniocentesis, cordocentesis.
A method of prenatal screening is represented by the triple test, which is performed in weeks 14-16 of pregnancy. For this test are determined from maternal serum:
- alpha-fetoprotein (AFP);
- beta-human chorionic gonadotropin (beta-hCG);
- unconjugated estriol.
Alpha-fetoprotein (AFP), a protein synthesized by the fetus is detectable in maternal serum from week 6 of pregnancy,with a peak in week 34 of gestation (4 mg / ml), its value decreasing in 8-12 months after birth. Measurement of alpha-fetoprotein can be done from amniotic fluid or from maternal blood.
Elevated values of alpha-fetoprotein are found in:
- Multiple pregnancies;
- Skin diseases;
- Organ failure;
- Congenital nephropathy;
- Cystic higroma;
- Hepatic necrosis;
- Neural tube defects (spina bifida, anencephaly);
- Abdominal wall defects.
Low values of alfa-fetoprotein are recorded in cases of:
- Chromosomal abnormalities;
- Defects of the placenta;
- Fetal hydrops;
- Trophoblastic disease;
- Diabetic mothers.
Beta-human chorionic gonadotropin (beta hCG) is a hormone secreted by sincitotrofoblast and is present in maternal serum after blastocyst implantation. Its level, in maternal serum, increase until week 10 of gestation and then begins to decrease until week 18, after which the level remains constant.
Unconjugated estriol is secreted by sincitiotrofoblast.
For trisomies are suggestive low levels of AFP and unconjugated estriol and increased levels of beat-hCG. In Turner syndrome, a monosomy, AFP and unconjugated estrolul are slightly low and beta-hCG is elevated, making this syndrome to be detected when a trisomy is suspected.
Fetal ultrasound examination is used to assess fetal age and evolution of pregnancy, placenta location, presence of twin pregnancies, the presence of birth defects (neural tube defects, cardiac anomalies, limb anomalies).
Fetal ultrasound examination has the advantage of being non-invasive and all harmless both for fetus and for mother. Is performed starting from week 8 of gestation, with periodic reassessment of fetal and placental parameters in weeks 16-18 of gestation.
First-trimester ultrasound examination is performed in the first 12 weeks of amenorrhea, for:
- Accurate determination of gestational age (in some cases there may be a discordance between the presumed age of pregnancy and appropriate age assessed by ultrasound measurements);
- Determining the number of fetuses;
- Pointing out morphological abnormalities;
Second-trimester ultrasound examination is performed through week 22 of pregnancy:
- Evaluates the fetus (cephalic diameter, abdominal circumference, femur length);
- Abnormality detection.
Third-trimester ultrasound examination is performed until 32 weeks for:
- Assessment of fetal development;
- Study fetal morphology because some abnormalities can be pointed out later in fetal development.
Ultrasound age is expressed on weeks of amenorrhea, from the first day of the last period. Better knowledge of development age is important to detect developmental abnormalities.
Ultrasound Screening For Down Syndrome Detection
Include measuring of cranio-caudal length, biparietal diameter, femur length, abdominal circumference, nuchal translucency, highlighting the ethmoid bone. Femur and humerus length in Down syndrome is low.
Nuchal translucency of 3 mm between 11-13 weeks of gestation is associated with a three times higher risk of having a newborn with trisomy 21 than the corresponding risk of maternal age. Absence of etmiod bone increases the risk of Down syndrome.
Biparietal diameter / femur length ratio and nuchal translucency measurement between 11-13 weeks of gestation and maternal age are used to determine the risk of Down syndrome.
Other ultrasound markers are:
- Echogenicity of the heart;
- Tricuspid regurgitation;
- Cystic higroma;
- Duodenal atresia;
- Cysts of the choroid leave;
- Dilated renal pelvis;
- Echogenicity of the bladder;
- Sandal gap.
Ultrasound Screening for Turner Syndrome Detection
- Massive edema;
- Cystic higroma;
- Fetal hydrops;
- Encephalocele / meningocele;
- Intrauterine growth retardation, short femur;
- Cardiac anomalies (coarctation of the aorta);
- Renal abnormalities;
- Hypoplastic placenta