What is prenatal diagnosis?
Prenatal diagnosis is the identification of a medical condition before an infant is born. This may be undertaken during pregnancy, or prior to implantation of an embryo during in-vitro fertilisation.
What is the difference between prenatal screening and prenatal diagnosis?
Prenatal screening is a means of assessing if a fetus is at increased risk of an abnormality. Screening is routinely offered to all pregnant women in developed countries, and is performed during the first and second trimester (up to 26 weeks of gestation). Screening involves a fetal ultrasound and maternal blood tests. Abnormalities screened for are those considered to have significant morbidity/mortality.
Prenatal diagnosis is the determination of a specific medical condition using genetic testing, immunohistochemistry and electron microscopy. As with prenatal screening, prenatal diagnosis is only indicated for conditions associated with significant morbidity/mortality.
Why is prenatal diagnosis undertaken?
Prenatal diagnosis is performed for the following reasons:
- Provides expectant parents with the choice to continue or have a termination of pregnancy in the setting of a known diagnosis of a severe inherited skin condition.
- Allows for optimal preparation for the birth and initial care of the neonate;
- Parents can be appropriately counselled and prepare themselves.
- Potential complications of the delivery can be anticipated, such as increased risk of failure to progress during labour in the setting of a fetus with X-linked ichthyosis.
- Delivery can be arranged to take place in a suitable facility with access to a Neonatal Intensive Care Unit.
What skin conditions can be diagnosed prenatally?
Skin conditions suitable for prenatal diagnosis include:
The causative gene mutation of many other inherited skin conditions are known; however, prenatal testing should only be performed for disorders with high morbidity/mortality.
Is determination of fetal sex important in prenatal diagnosis of skin conditions?
The sex of the fetus can be very important in prenatal diagnosis of inherited skin conditions. Incontinentia pigmenti is an X-linked dominant condition that is usually fatal in male fetuses, and affects females to varying degrees. X-linked ichthyosis results in skin disease in males; female carriers are unaffected or may develop asymptomatic corneal opacities.
Methods and timing of prenatal diagnosis
The tests used for prenatal diagnosis are described in the table below.
|Method||Skin disorders tested for||Timing||Technique|
|Pre-implantation genetic diagnosis||Harlequin ichthyosis
Severe ectodermal dysplasia syndromes
|3 days after in-vitro fertilisation||Genetic testing of embryos after fertilisation. Embryos confirmed to be free of the disorder are implanted in the uterus.|
|Chorionic villus sampling||Epidermyolysis bullosa
|Weeks 10–13 of pregnancy||Cells from placental villi are collected using a needle inserted either through the cervix or trans-abdominally under ultrasound guidance. Fetal DNA is extracted and analysed for a known genetic mutation.|
|From week 14 of pregnancy||Fetal cells are collected from the amniotic fluid by a needle inserted trans-abdominally under ultrasound guidance. Fetal DNA is investigated for a known genetic mutation.|
|Fetal skin biopsy||Skin conditions with unknown genetic mutation||From week 19 of pregnancy (once fetal skin adequately formed)||Punch biopsy of fetal skin is performed under ultrasound guidance. The biopsy is examined by immunohistochemistry and electron microscopy.|
|Maternal serum screening||X-linked ichthyosis (identified by low levels of Ue3)||During second trimester of pregnancy (weeks 14–26)||Maternal serum levels of specific hormones and proteins are measured by a blood test to assess the risk of different conditions.|
|Ultrasound||Harlequin ichthyosis (suggested by echogenic amniotic fluid, joint and digital contractions plus facial dysmorphism)||From 12 weeks of pregnancy onwards||Trans-abdominal ultrasound performed to assist with diagnosis. Useful in identifying conditions with no known family history.|
What are the potential complications of prenatal diagnosis?
The potential complications of prenatal diagnosis are specific to the technique used:
Pre-implantation genetic diagnosis
- Risk of misdiagnosis: technically difficult and small cell number available to use for genetic analysis
- Lower pregnancy rates compared to IVF without prenatal diagnosis
- Very costly
CVS, amniocentesis and fetal skin biopsy
- Small risk of fetal loss (1–3%)
- Amniotic fluid leakage
- Rhesus blood group sensitisation in a Rhesus-negative mother
Fetal skin biopsy
- Scarring (usually minor) of newborn
- Inconclusive results if performed too early (prior to adequate fetal skin formation)
Can treatment of the skin condition be given prior to birth?
There is currently no means of treating inherited skin conditions prior to delivery of the infant. Further advances in fetal medicine may make this a possibility in the future.