Cystic Fibrosis and Antenatal Screening

The Leeds Method of Management. April, 2008. Cystic fibrosis and Antenatal Screening [online]. Leeds Regional Adult and Paediatric Cystic Fibrosis Units, St James's University Hospital, Leeds, UK. Available from http://www.cysticfibrosismedicine.com [online].

Antenatal screening (before conception)
Antenatal diagnosis (diagnosis before birth)
Echogenic bowel
Chorionic villous sampling
Amniocentesis
Other options for a couple known to carry CF causing mutations

Antenatal screening (before conception)

Antenatal screening for CF causing mutations in people of child bearing age has been studied both in the hospital antenatal clinic (Brock, 1996; Gilfillan et al, 1999) and general practice (Harris et al, 1992). Couple screening when the parents are notified only if both are found to have a CF mutation appears to be a satisfactory method that avoids unnecessary anxiety (Miedzybrodzka et al, 1995; Murray et al, 1999). This is not widely available on the NHS but is available privately.

Antenatal diagnosis (diagnosis before birth)

The identification of the CFTR gene in 1989 made reliable antenatal diagnosis possible using chorionic biopsy at around 11 weeks gestational age or amniocentesis at about 16 weeks.

There are three main situations where the antenatal diagnosis of CF is considered:

1) Parents who already have a child diagnosed with CF. Parents of a child already identified with CF have a 1:4 chance, in each subsequent pregnancy, of having a further child with CF. It is common for such parents to want antenatal testing of future pregnancies.

2) Parents identified as carriers of CF causing mutations.

3) When a foetus is identified as having an echogenic bowel (a very bright bowel on antenatal ultrasound).

This can occur for many reasons but about 1-3% of these foetuses will have CF. If echogenic bowel is seen the parents are given the option of antenatal diagnosis or carrier testing. If both parents are found to be carriers the chance of the foetus having CF is greater than 1:4 and antenatal testing will be offered to confirm or refute the diagnosis.

Parents of a child with CF who are planning to have more children and who wish to have antenatal tests performed to determine if the foetus has CF should ensure they have an appointment to see the geneticist BEFORE embarking on the new pregnancy. An appointment will be arranged by the CF Clinic staff on request. It is very important to realise that CF cannot always be excluded by genetic tests alone.

With the widespread introduction of neonatal screening it is probable that more individuals of child bearing age will be aware that they carry a CF causing mutation. This will probably lead to partners requesting tests to establish carrier status and then antenatal diagnosis of at risk pregnancies.

Chorionic villous sampling (CVS)

Providing it is possible to access the placenta this test is performed at around the 11th week of the pregnancy. The test results are usually available in one to five days and may confirm or exclude the diagnosis of CF. This antenatal test carries a small risk to the baby (1%) and should only be performed if discovery of an affected foetus would result in termination or a medical induction. Most people electing to have antenatal CF testing opt for CVS.

Amniocentesis

This test is performed at about the 14th week of the pregnancy. Foetal cells can be aspirated from the amniotic fluid that surrounds the baby. Foetal cells need to be cultured for two to three weeks before sufficient DNA is available to be tested. Results usually take three to four weeks to return. There is a very small risk of miscarriage after the test (0.7%).

Other options for a couple known to carry CF causing mutations

Pre-implantation diagnosis is at present only available at specialist centres and limited by funding and the in-vitro fertilisation success rate. Eggs and sperm are harvested from prospective parents who have already had a child with CF. Following in-vitro fertilisation the developing embryos are screened at a very early stage for their CF status. Selected embryos, free of CF, are implanted into the mother's uterus to continue their development.

 

Key points

• Chorionic villous sampling can be performed very early in the pregnancy and results are usually available quickly so that a vaginal termination is possible

• Amniocentesis is available at around 16 weeks and results take between three to four weeks. If the result is positive a medical induction may be offered

Recommendations

• Genetic counselling should be offered to all at risk couples prior to pregnancy

• Parents of a child with CF who are planning to have more children and who wish to have antenatal tests performed to determine if the foetus has CF should ensure they have an appointment to see the geneticist BEFORE embarking on the new pregnancy

• Any couple presenting with an early pregnancy who are at increased risk of having a child with CF should be referred urgently for counselling

 

References

Brock DJ. Prenatal screening for cystic fibrosis: 5 years’ experience reviewed. Lancet 1996; 347: 148-150. [PubMed]

Gilfillan AJ, MacKenzie IM, Warner JP, et al. Antenatal screening for cystic fibrosis: the Edinburgh experience. Netherl J Med 1999; 54(Suppl): S32.

Harris HJ, Scotcher D, Craufurd D, et al. Cystic fibrosis carrier screening at first diagnosis of pregnancy in general practice. Lancet 1992; 339: 1539. [PubMed]

Miedzybrodzka ZH, Hall MH, Mollison J, et al. Antenatal screening for carriers of cystic fibrosis: randomised trial of stepwise v couple screening. BMJ 1995; 310: 353-357. [PubMed]

Murray J, Cuckle H, Taylor G, et al. Screening for cystic fibrosis. Health Technol Assess 1999; 3: 1-104. [PubMed]

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