There are two methods for obtaining fetal DNA for diagnostic testing available today – a placental biopsy, also known as a chorionic villous sample (CVS), and an amniocentesis.
Chorionic Villous Sample (CVS)
A CVS is technique for obtaining fetal tissue via a placental biopsy typically between 10-14 weeks gestation (although this technique may be used at more advanced gestational ages under certain circumstances). When undergoing a CVS, one of two approaches is undertaken – a transabdominal or transcervical approach. In a transabdominal approach (usually performed if the placenta is on the front of the uterus) a biopsy needle is guided with real time ultrasound through the lower anterior abdominal wall of the mother into the placenta where a microscopic sample of placental tissue is obtained. With a transcervical approach (usually performed when the placenta is on the back of the uterus), a small flexible catheter is guided through the cervix with the aid of real time ultrasound to obtain a microscopic sample of placental tissue. The amount of tissue (10-20 milligrams) obtained is insignificant to the developing fetus and provides ample DNA for any diagnostic test.
From an embryologic standpoint, the placenta and fetus are genetically identical and therefore sampling cells from the placenta provide the same genetic information as sampling the fetus directly (there are rare exceptions…. but they are very rare)
An amniocentesis is the second diagnostic technique usually performed between 15-20 weeks gestation. As the fetus develops, it begins urinating – and at about the same time – swallowing. The fetus produces urine that is then swallowed, absorbed in the gastrointestinal tract and effectively exchanged for fresh fluid from the maternal circulation across the placenta. This cycle is continuous. When an amniocentesis is performed, a small amount of amniotic fluid (fetal urine surrounding the developing baby) is sampled with a very thin needle under the guidance of real time ultrasound. Contained in this fluid are cells that have sloughed off the genitourinary tract (kidneys, ureters and bladder), skin and lungs. These cells are collectively called “amniocytes”. Since all cells carry the same genetic information, it doesn’t matter where they originated – and they contain fetal DNA that can be analyzed in a diagnostic test.
Risks associated with invasive procedures – a perinatologist’s perspective
Aversion to an invasive procedure is understandable. However, most of the aversion is probably based on misinformation outdated data and a lack of informed counseling. This is not to say invasive procedures should not be taken seriously or without proper counseling and informed consent, but the bad reputation they have garnered compared to their benefit is unfortunate.
The most common fear of an invasive procedure is pregnancy loss. Absolutely. It is a risk that must be stated, however, that risk is overstated significantly. The internet, social media groups and printed material have convinced patients that the loss rate is as high as 1%. If this loss rate were true, we would not perform a single procedure. Would you get on a plane that had a 1% chance of crashing? I wouldn’t. While the state of California’s compiled data (from all invasive procedure providers in the state of California) put’s the loss rate at “less than one in a thousand” – our practice has not experienced a single loss related to an invasive procedure.
Ultimately, pregnancy carries with it risk – maternal and fetal. Managing those risks is really the objective. For example, by not undergoing and invasive procedure and diagnostic test (say a microarray) a woman forgoes the incredibly small risk of pregnancy loss but incurs the 1.5-2% risk of having a child with a microarray abnormality. Risk cannot be avoided, just managed….and ultimately each individual/couple must choose which risk they want to incur.
Who should consider a diagnostic procedure?
As recently as a few years ago, this was a pretty easy answer. A woman should consider a diagnostic procedure if:
- she is 35 years of old or older
- she had a chromosome abnormality in a previous pregnancy
- she has a family history or are at risk for a specific genetic disorder
- she has received abnormal results from a screening test
But just as computer technology has changed – so too has the ability to detect abnormalities with genetic testing. Today we understand that every pregnancy is at significant risk for a genetic abnormality regardless of maternal age. Microarray abnormalities (small deletions or duplications of the genetic code) occur about 10 times more frequently than Down syndrome in the general population (1/65 vs 1/650). With that knowledge – every woman should discuss the risk of a genetic abnormality with her primary obstetrical care provider or schedule a genetic counseling appointment in our office to understand the risks in every pregnancy and the testing options available.
A Perinatologist's Perspective:
The nuchal translucency (NT) exam at 11-14 weeks and the detailed is an absolutely critical exam. It not only provides early information about the health of the pregnancy, but also accurately dates the pregnancy - and while many people are sure of their due date before the NT exam, statistics show that about 60% of women do not get their conception date correct (and thus their due date). It may not seem like an important matter at 12 weeks, but having a due date off by 5-7 days 28 weeks into the future could be critical to the timing of your delivery and health of your baby.