The second trimester begins at about 13 weeks and continues until the twenty-seventh week. What make this trimester unique is that it spans a time period where a pre-viable fetus becomes a potentially viable neonate, albeit premature{pre-term.} At exactly what week or day or fetal/neonatal weight this transformation occurs depends on many factors and "unknowns." In current practice, 1995, it is generally felt that a preterm birth prior to 24 weeks is unlikely to survive, although it is not impossible and without hope. [when I was a Ob-Gyn resident in 1973, 28 weeks was considered the critical gestational age.] The third trimester shares similar etiologies to the mid or second trimester.  

This month, I will discuss chromosomal abnormalities as they account for nearly 15 per cent of mid-trimester pregnancy losses.  

Fortunately, there exists for all pregnant women an opportunity for a comprehensive prenatal evaluation of their fetus for the purpose of determining genetic and morphologic[structural] normalcy. Although it is not considered mandatory or routine, women at risk for fetal problems may avail themselves to several testing procedures as well as be offered screening procedures. Because some tests are invasive e.g.. amniocentesis, CVS, fetal blood sampling and direct fetal visualization (fetoscopy,) it is important for the patient and her physician to have a clear understanding of what information is to be ascertained from a particular test, its risks and its limitations. Other tests such as blood screening and high resolution ultrasound are less invasive. With these guidelines well defined, the prenatal diagnosis of fetal abnormalities becomes an important tool in the contemporary care of pregnancy.  

Pre-pregnancy counseling is very important, and women should consult their physician or other primary caregiver prior to conception, or at least very early in the first trimester, so as to determine any risk factors for genetic diseases; e.g. familial disorders such as cystic fibrosis, muscular dystrophy, hemophilia, sickle cell anemia, Tay-Sachs Disease or chromosomal disorders, the most common being Down's Syndrome or Trisomy 21.  

Carrier states can then be detected prior to conception or early in the pregnancy. Appropriate diagnostic tools may be utilized as described above: blood sampling, chorionic villus sampling, amniocentesis and high resolution ultrasound. For families who have been identified at risk for these or other hereditary diseases, genetic counseling is indicated. Genetic centers are usually integrated in University centers and I will be adding a national resource list of these sites for reference.(Available in the Hygeia Reference area.)  

For families not at risk for genetic disorders, certain screening tests(non-invasive) are available. The maternal Alpha Feto Protein test(now combined with two other markers-HCG and Estriol) is a screening test primarily for two particular classes of disorders; neural tube defects(NTD) and chromosome abnormalities, the most common being Downs Syndrome or Trisomy 21. If the test should return as "positive," either above or below the limits of the mean, it does not necessarily indicate there is a problem-only that their might be a higher risk for a problem. The next procedural step would be to perform a detailed ultrasound exam, sometimes called a "level 2" scan and possibly an amniocentesis. These second line tests can better define the health of the fetus within the limitations of each test.  

Genetic testing today, although available now for many years, is on the cusp of a new frontier-that of very early, embryonic diagnosis via micro-fetal blood sampling and maternal blood sampling for fetal cells and fetal chromosome analysis. Early fetal treatment may also be possible through current and ongoing research in gene manipulation and therapy.  

Genetically abnormal embryos that develop and survive the first trimester can be either diagnosed by the aforementioned prenatal tests, or be manifest by spontaneous labors or by the anguish of intrauterine fetal demise. One example of an abnormal chromosomal arrangement which presents with either premature rupture of the fetal membranes, premature labor, or fetal demise is Trisomy 18. Although this can be detected by CVS, amniocentesis and/or ultrasounds, most patients not at risk will not have these procedures. Maternal serum AFP testing can screen low risk populations but it is only a screening tool thus some patients might not avail themselves of it and others might have a false negative report. Trisomy 18 is not compatible with life and early diagnosis can offer affected parents the option for elective termination.  

Voluntary termination of a genetically abnormal fetus in the mid-trimester when selected as an option for the pregnancy is no less anguishing than spontaneous pre-viable miscarriage or delivery, for it not only involves loss but requires conscious decision making at a time when one's mind is replete with emotions of remorse. Patients at this time need careful, caring and informative counseling to help with their decision making process. If at all possible, counseling with a geneticist is the ideal. As difficult as are the decisions associated with situations of fetal death or non-viability, more so are those diagnoses involving anomalous chromosomes or handicapping physical abnormalities in a fetus who will be born alive and survive. It is not my intention in this forum to be judgmental or express opinions about the ethics of any options a physician might offer a patient veiled with these onerous problems, but these patents, mothers and fathers together crave information and guidance. Again, all options must be discussed and offered in a venue of information, compassion and objectivity. 

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