May 24, 2013

What Genetic Screening is Available After a Woman is Already Pregnant?

Previous FCNE blogs have reviewed genetic screening of embryos before they are implanted within the uterus; this is known as preimplantation genetic diagnosis or PGD.  What genetic screening is available after a woman is already pregnant and when are these recommended?

New technology has recently permitted noninvasive prenatal testing for fetal chromosomal abnormalities from a simple blood test.  Fetal DNA created within the placenta escapes the uterine environment and can be detected within the blood or plasma of a pregnant patient as early as 10 weeks gestation.  The technology uses a technique labeled massively parallel genomic sequencing.  This assay can assess millions of DNA fragments and is less time consuming than older techniques.  Results are typically available within one week.  This fetal DNA blood test potentially prevents a more invasive screening using amniotic fluid obtained by amniocentesis.  In an amniocentesis, a needle is placed through a woman’s abdomen and through the uterine wall to obtain the amniotic fluid.

Not all screening tests are full proof and most are with both false positive and false negative readings.  Fetal DNA testing is reported to identify 98% of trisomy 21 (Down syndrome) cases with a false positive rate of less than ½ percent.  Trisomy 13, trisomy 18 and sex chromosome anomalies can also be detected.  Testing has been done in a small number of twin and triplet gestations (Prenatal Diagnosis Volume 32 (8): 730–734, August 2012) but these small numbers do not yet permit routine use of the test in multiple gestations.

The American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women be offered screening for fetal chromosomal abnormalities; it does not recommend that all women require serum fetal DNA screening.  In a December 2012 ACOG Committee Opinion, indications for screening are reported as shown in Box 1 below:

At the Fertility Centers of New England, pregnancies are monitored until the 7th to 10th week of gestation.  Thereafter, a patient is referred to one of the many excellent obstetricians in your area who will further discuss appropriate prenatal screening for you.

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Medical Director,

Medical Director, Fertility Centers of New England