Prenatal Diagnosis: some details

Just briefly, here are some of the details about prenatal screening and diagnosis...

Maternal Serum Screen

Also known as the Triple Screen, this 2nd Trimester maternal blood test looks at levels of alpha-fetoprotein (AFP), betaHCG, and unconjugated estriol (uE3) in maternal serum. Some centers also look at inhibin level and pregnancy associated plasma protein A (PAPP-A). (They can also check some of these levels in the first trimester, and integrate the results with the Triple Screen.) The first important point is that these tests are not diagnostic tests. They simply try to quantify the statistical likelihood or risk that the baby has one of three disorders: Down Syndrome (Trisomy 21), Edward's Syndrome (Trisomy 18) and open neural tube defects like spina bifida.

It is important to realize that this test, while posing no physical risk to the mother, does not get her any closer to knowing whether or not her baby has one of those three disorders. In fact, using the 2nd Trimester maternal serum screen by itself for Down Syndrome, the false positive rate is about 7%. If you screened 1000 women, 70 will have a "positive test" and only one baby will have Down Syndrome. (There are also, of course, false negatives.) The very real risk, here, is that 69 women will have tremendous psychological stress and worry for no reason. Many of them will move on to have amniocentesis for no reason. Amniocentesis carries a 1 in 200 risk of miscarriage, among other less common risks.

Other tests in what's called the IPS of integrated prenatal screen include 1st trimester serum levels (mentioned above) and a Nuchal Translucency (NT) ultrasound. The NT is done in week 11, 12 or 13, and measures the thickness of the fold at the back of the developing baby's neck, which is correlated with an increased risk of Down Syndrome. This is not a diagnostic test either. In combination with the serum tests it can help lower the overall false positive rate.

Amniocentesis and chorionic villus sampling (CVS) are actual diagnostic tests. A long needle is placed through the abdomen to sample the amniotic fluid or the chorion. Cells from the fluid can be analyzed to determine the baby's sex and presence of all sorts of genetic abnormalities. (The price goes up for each abnormality tested for). After one of these tests one can have a fair degree of certainty as to the presence or absence of a genetic or chromosomal disorder. Amnio has a 1/200, and CVS a 1/100 risk of miscarraige.

On the other hand, a routine ultrasound to check baby's anatomy at around 18 weeks can pick up an open neural tube defect in most cases.

Can you do anything about these disorders? Certainly not Down's or Edward's - no cure for these. For isolated menigomyelocele, a type of open NTD, there is promising research underway showing that intrauterine surgery, between weeks 24 and 30, can be of some benefit when compared to postnatal surgery. However, there are a lot of risks attached to surgery in utero (death being one of them), so that this surgery is not offered routinely.

The conclusion is that a routine 18 week ultrasound for anatomy will pick up many abnormalities in a baby (open NTDs, polycystic kidney disease, gastroscicis, etc.). Surgery, in the future, may be a routine option for some of these, but not really at the moment. As far as I can see, there is no reason to increase stress and worry with a Maternal Serum Screen if you are not going to abort the baby or risk miscarraige from amniocentesis or CVS.

Ultrasound
As far as the safety of ultrasound goes, I know of two large, randomized controlled trials, that demonstrate the safety of ultrasound in pregnancy. Keiler et al, in BJOG, 1997, looked at 4637 randomized pregnancies and found no reduced hearing, no reduced vision, and no difference in height/weight at ages 1, 4 and 7. Ultrasounds have been around for almost two generations, and there has yet to be a study showing detrimental effect. On the other hand, as Leivo et al. showed in a study of 9310 women, perinatal deaths were reduced in the ultrasound group (deaths around the time of delivery). It is a safe and cost-effective means to be reassured about the health of the baby instead of rushing of to a C-Section. It's also been shown to significantly increase parental bonding, as well as objectively reducing pregnancy-related anxiety levels.