When an individual is found to have a structural aberration the rule is to obtain the karyotype of his parents and if necessary of the siblings: first to find the origin of the defect and second to inform family members of the reproduction risk if themselves are normal physically but carriers of a translocation (fig 2). We also suggest to karyotype couples who have a personal or familial history of repeated foetal losses or birth of children with mental retardation with or without malformations or dysmorphism.
 

I- 5. Hereditary metabolic diseases
a-metabolic disease diagnosed in a child and finding of an enzyme deficiency or a mutation that could be detected in a subsequent pregnancy.
b-known metabolic disease: the study of the parents show that they are both carriers (heterozygous) and that they have a 25% risk of conceiving an affected child like in cystic fibrosis. Due to the high gene frequency of this disease screening programs to detect carriers are now being offered in some high risk populations.

I- 6. Neural tube defects
Neural tube defects have a multifactorial etiology and their incidence is widely variable. They used to be more frequent in the British Isles, Canada, China and other countries like Hungaria with an incidence of 5/1000 births and a recurrence risk of 5%. In France and United States the incidence was more like 1/1000 with a low recurrence risk.
It has been shown, first in Great Britain, that the intake of folic acid may help the closure of the neural tube and lower the recurrence frequency in high risk pregnancies. Closure of the neural tube takes place during the first 4 weeks of embryogenesis. The intake of folic acid as soon as a pregnancy is planned, and for the first two months has reduced the incidence and recurrence of the neural tube defects.
It is highly recommended when there is a family history of neural tube defect to monitor a pregnancy in ultrasonography to make sure that both foetal cranium and rachis are normal. In families at risk it is very important to encourage the intake of folic acid as soon as a woman plans a pregnancy or cease all means of contraception.
Spina bifida aperta: opening of the tissues over the bone defect with or without extrusion of the meninges; it is occulta if the skin is normal.
Anencephaly: closure defect of the cranial vault. The skull defect may be limited to a region of the cranium and be variable in size, we then refer to an encephalocele
Open lesions of the neural tube and cranium induce an elevation of alpha-foetoproteins in amniotic fluid and subsequently in maternal serum .
If there is a history of previous or familial microcephaly a family study will help determining if this is a hereditary defect: autosomal dominant or recessive. Ultrasound examinations of the developing foetus may facilitate the follow up of the head bi-parietal diameter and circumference, but only severe growth deficiencies will likely be detected.

II- Foetal Ultrasonography

II- 1. Ultrasonography (Fig 3) makes use of ultrasounds to study tissues and organs. It is applied from the first trimester but it is only during the second trimester that one can best evaluate foetal morphology and preferably around the 18^th week of gestation.

II- 2. Cardiac ultrasonography, that allows examination of great vessels and heart chambers, is done usually around the 20^th week of pregnancy.

II- 3. Markers suggesting the presence of a birth defect.
Ultrasonographic markers are variations observed during the ultrasound session that will alert the examiner to the possibility of an abnormal foetal development or a genetic disease. Those markers can reveal for example the possibility of a chromosomal anomaly such as a trisomy 21, 13, 18, or a chondrodysplasia.
 

ULTRASOUND MARKERS SUGGESTING THE PRESENCE OF A FOETAL ANOMALY

II- 4.1 Nervous system anomalies

II- 4.2 Cardiovascular defects

II- 4.3 Thoracic anomalies

II- 4.4 Gastro intestinal malformations

II- 4.5 Urogenital malformations

II- 4.6 Musculo skeletal malformations

II- 4.7 Other anomalies

III- 1. Amniocentesis
The amniocentesis (fig 4) is early if done around the 12^th week of gestation. Today several prenatal diagnostic clinics perform amniocenteses between the 14th and 16^th week of pregnancy. Studies have shown an increased loss of amniotic fluid if the amniocentesis done before the 12^th week and there is a risk of skeletal anomalies in particular of club feet secondary to oligoamnios. According to the age of pregnancy from 10 to 30 ml of fluid are obtained during the procedure ( fig 5). Foetal cells from the upper digestive system, urinary tract, skin and membranes are found in the fluid and recuperated by centrifugation of the specimen. They are then kept in culture for a period of 5 to10 days in a culture medium to which calf serum has been added. Cellular multiplication is then sufficient and allows the preparation of microscopic slides allowing the numerical and structural studies of the metaphasic chromosomes. Treatment of chromosomes during the slide preparation reveals segments of different intensity or banding patterns. Those bands reflect a variable ratio of AT;GC nucleotides on the chromatids and help to identify chromosome pairs.

III- 2. Chorionic villus sampling (CVS)
The biopsy or aspiration of chorionic villi by the vaginal route (fig 6) yields foetal cells, several of which are in the process of dividing and can be analysed during the hours following the procedure. There is a risk of miscarriage and maternal cell contamination of the specimen thus leading a number of clinicians to abandon this procedure done before the 12^th week of pregnancy. Reduction limb defects have been reported if the CVS is done towards the end of the first trimester. In special circumstances when the risk of genetic disease is high as for instance in hereditary metabolic diseases or if one of the parents is carrier of a balanced chromosomal translocation, this technique has the advantage of reaching a diagnosis around the 11^th or 12^th week of gestation.

III- 3. Cordocentesis
Blood can be obtained from the foetal cord under ultrasound guidance. If at the end of second trimester there is an urgent need to confirm a diagnosis or to avoid extraordinary measures if there is there is a threat of premature labor and the foetus is found to be abnormal This procedure will allow a short term chromosomal analysis from lymphocytes or an enzyme study. The rapid cytogenetic study could also confirm or exclude a chromosomal defect previously found in the amniocytes. This approach can also be useful to delineate mosaicism like for instance a trisomy 20 which usually has a favourable outcome or identify a chromosomal marker confined to annexial tissues. Cordocentesis has been used to study severe immunological disorders by measuring adenosine deaminase and doing T cells analysis.

III- 4. Foetoscopy
Foetoscopy is a technique that allows to visualize the foetus around the end of the second trimester, by introducing a tube with optic fibres through the abdomen and the uterus allowing to biopsy the foetus or proceed to surgical interventions. For security reasons this invasive technique is not a routine procedure and is rarely utilized except in development programs.

III- 5. Fetal cells in maternal circulation
The presence of foetal cells in the maternal blood stream could give us some information of the chromosomal complement and the foetal genotype. Research in this direction has been initiated several years ago but up to date has yielded meagre results on the efficacy of this non invasive procedure. Several difficulties are encountered: the low frequency of nucleated cells, means to isolate them, their identification and genetic analysis. The venue of FISH technique and other means to identify abnormal chromosomal complements, and the PCR (polymerase chain reaction) for molecular analysis have recently convince researchers not to abandon this avenue which could be a great asset to the prenatal diagnosis of genetic disease.

IV- Congenital anomalies due to single gene defects

Several congenital malformations have a hereditary origin and their mode of transmission can be autosomal dominant, recessive or X linked. Some syndromes could be identified during the second trimester of pregnancy if a major anomaly is detected at ultrasound ( like bone dysplasias). However if there is a history of congenital anomaly of unknown etiology but seen in a previous pregnancy, the mother could be reassured at ultrasound if she is pregnant again. This measure applies for instance in cases of omphalocele and laparoschisis for which the risk of recurrence is low.

V- Maternal serum markers

Serological markers are normal proteins found in the maternal circulation and if their level is abnormal it will allow the detection of some foetal pathologies early in pregnancy. The screening efficacy is increased if both procedures foetal ultrasonography and marker studies are completed simultaneously.

V- 1. Neural tube defects
Alpha-1 foetoprotein (AFP) constitute 20% of circulating foetal proteins and its level varies with foetal age at the time it is measured (fig 7). An increase of AFP in the amniotic fluid will reflect on the maternal serum level and may alert to the presence of an open neural tube defect. A higher than normal level in the amniotic fluid may be due to a skin lesion unseen at ultrasound or may be due to foetal demise.

V- 2. Trisomy 21 or Down syndrome

VI- Fluorescence in situ hybridization (FISH)

Fluorescense in situ hybridization makes use of molecular probes marked in fluorescence that correspond to a gene or DNA sequence and showing a bright signal under ÙV microscope at a specific locus on a chromosome. First the FISH technique can apply to interphasic cells readily obtained at amniocentesis and confirm the presence of an euploid or aneuploid complement like for chromosomes X,Y, 13,15,18,21 . This technique also applies to chromosomal markers that can be detected in amniocytes with the help of FISH. The technique can also be used to identify the origin of supernumerary segments or confirm the loss of specific sequences or a deletion on a given chromosome.

VII- Future perspectives

VII- 1. In utero treatment
The in utero treatment is limited to fluid aspiration for instance in bladder hypertrophy secondary to an urethral valve, and in some thoracic or abdominal fluid collections. Other surgical procedures have been completed like the insertion of a catheter in the bladder, correction of certain heart defects, to name the most common.
The finding of an abdominal wall defect ( ex: omphalocele) allows time for planning the mode of delivery and alert neonate specialists and pediatric surgeons to an immediate intervention at birth to prevent systemic complications.
As part of indications for medical fœtal therapy, fœtal arythmia can be corrected via a maternal medication approach. Finally a restrictive phenylalanine diet, prescribed as soon as a phenylcetonuric woman plans a pregnancy, will prevent foetal complications and especially a microcephaly and severe mental retardation in the child.
The prenatal diagnosis of some metabolic diseases like galactosemia or leucinosis will allow to plan ahead if the foetus is sick and prescribe a restrictive metabolic milk for the infant to prevent early complications secondary to abnormal metabolites.

VII- 2. Preimplantation diagnosis
Preimplantation diagnosis is defined as the analysis of a cell taken from a fertilized egg at, for instance, the eight cell stage. It was introduced as an assisted procreation technique in 1989 but it still considered a research and development procedure. We do not know yet if it is a secure and reliable procedure although to date but according to literature reports a few dozens infants born after this technique seem to have a normal development. A limited number of laboratories in Europe and America have the facilities and the knowledge to offer this test as a diagnostic procedure.
to date this technique has been attempted in more than one hundred pregnancies at risk and especially in mucoviscidosis, Duchenne muscular dystrophy, hemophilia A, alpha and beta thalassemia, bulbar atrophy, Lesch Nyhan syndrome, incontinentia pigmenti, Huntington disease, myotonic dystrophy.
For X linked diseases fœtal sexing is recommended before proceeding to a molecular diagnosis.

II- 3. Preconception screening
Preconception screening is a recent avenue in the field of prevention. It implies the detection of gamete anomalies and more specifically at the ovum level. Recent publications refer to preconception diagnosis by studying the first polar body in maturing ova: polar bodies reflect the chromosomal complement and genotype of the ovum that could be used in the in vitro fertilization. In a recent literature report, a normal ovum was selected from a mother known as a carrier of a dominant gene for a severe form of Alzheimer disease. It was reported that this ovum free of mutation was used for the in vitro fertilization and a normal infant was carried to term. The route is then traced for diagnostic interventions based on gamete studies, but ethic dilemmas will surge from any attempt to manipulate germinal cells.

Contributor(s)

Written	06-2002	Louis Dallaire

Citation

This paper should be referenced as such :

Dallaire L . Prenatal Diagnosis. Atlas Genet Cytogenet Oncol Haematol. June 2002 . URL : http://AtlasGeneticsOncology.org/Educ/PrenatID30055ES.html

© Atlas of Genetics and Cytogenetics in Oncology and Haematology

indexed on : Thu Apr 17 13:58:24 2008

For comments and suggestions or contributions, please contact us