New horizon: foetal DNA testing
IN the majority of cases, most pregnancies result in a healthy mother and baby. Chromosomes are the genetic make-up of an individual and every individual has 23 pairs of chromosomes and a pair of sex chromosomes which are received from both parents at the time of conception.
On some occasions, there may be an extra copy of a chromosome resulting in a trisomy. When there is an extra copy of chromosome 21, this is known as Down’s Syndrome; chromosome 18, Edward’s Syndrome; and 13, Patau Syndrome. These syndromes can result in delayed development, poor growth and feeding, underlying cardiac and neurological problems of the infant and neonatal/infant death.
A diagnosis of these conditions prior to delivery can mentally prepare the parents so they are not surprised at the time of birth. Screening tests for chromosomal abnormalities in the second trimester include the ‘quad screen’ which analyses four hormone levels that, when elevated/decreased, are associated with chromosomal abnormalities. When these are high, an amniocentesis is offered where a sample of amniotic fluid is extracted via ultrasound guidance and sent to the lab for analysis. This fluid contains foetal DNA that are diagnostic of abnormalities.
In the first trimester, Chorionic Villus Sampling (CVS) is offered which takes cells from the placenta that contains foetal DNA. Both tests are associated with adverse outcomes including foetal loss but both are accurate. Foetal cells which contain foetal DNA can enter the maternal circulation from as early as 10 weeks gestation via the placenta. It is estimated that two to six per cent of the DNA in the maternal blood is of foetal origin. These cells can remain in circulation for a long time but usually start disappearing two hours after delivery.
Foetal DNA fragments are much smaller than maternal DNA, so it is possible to distinguish their difference based on size. Where am I going with this? Nowadays it is possible to detect chromosomal abnormalities from as early as 10 weeks with a simple blood test from the mother (cell free foetal DNA or cffDNA). The test is extremely accurate with a false positive rate of less than 0.1 per cent. In addition, the sex of the foetus can be determined with 99 per cent accuracy (the sex is usually determined via ultrasound at around 18-20 weeks).
It must be stressed that this test is a screening test and if it comes back positive for chromosomal abnormalities, invasive tests such as amniocentesis and CVS should be done. cffDNA also does not detect spinal cord abnormalities and is non-specific for multiple gestations thus further screening with an ultrasound in the second trimester should still be done.
cffDNA is available in Jamaica and is called the Harmony Prenatal Test, offered by CARIGEN at the University of the West Indies. To date the lab has tested approximately 60 patients with good effect.
I would recommend that all patients do this screening test as chromosomal abnormalities can occur regardless of maternal age groups. This is the future of foetal screening and it is now readily available to us in the Caribbean. Speak with your obstetrician-gynaecologist about this test. As time evolves and technology advances, it will probably be able to fully replace CVS and amniocentesis.
Dr Daryl Daley is a medical officer in obstetrics and gynaecology at May Pen Hospital, and consultant OBGYN at Gynae Associates, 23 Tangerine Place, Kingston 10, and Shops 46-50, Portmore Town Centre, St Catherine. Telephone him at 929-5038/9 and 939-2859 or e-mail drdaryldaley@gmail.com.
