Tests During Pregnancy

ACOG (American College of Obstetricians and Gynecologists) has updated two committee opinions on carrier screening. Committee Opinion 691 reviews the recommendations based on disorders. Committee Opinion 690 addresses the issues related to use of screening strategies such as expanded gene panel testing.

Changes to look for compared to previous guidance include the following:

• Spinal Muscular Atrophy (SMA) has now joined cystic fibrosis (CF) as a recommendation for all women who are pregnant or considering pregnancy
• Hemoglobinopathies
  • Test all patients for CBC and RBC indices as part of antepartum care (ideally preconception)
  • Add Hgb electrophoresis if:
    • Increased risk based on ethnicity:  African, Middle Eastern, Southeast Asian, West Indian and Mediterranean ancestry
    • MCV is less than 80 fL with normal iron studies
• Ashkenazi Jewish Testing (central and Eastern Europe descent)
  • Recommended testing remains for 4 disorders
    • Canavan; CF; Familial dysautonomia; Tay Sachs Disease
  • Additional tests to ‘consider’ has been expanded to the following
    • Usher syndrome, Familial hyperinsulinism, Joubert and Maple syrup urine disease in addition to Bloom/Gaucher/Fanconi anemia/ML4/Neimann-Pick disease
• Tay Sachs Disease
  • In addition to Ashkenazi Jews, offer if either partner is French-Canadian descent or Cajun
  • Screening can be performed using DNA-based testing (mutation analysis) or hexosaminidase enzyme in serum or leuckocytes (leukocyte only with oral contraceptives)
  • Enzyme testing picks up approximately 98% of carriers regardless of ethnicity
  • Mutation analysis is highly effective in at risk populations – detection rate is limited in other populations
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In contrast, the NHS has no position on carrier screening for all patients.  Instead, they suggest talking to your GP is there is a history in your family. 

Speak to your GP if you're planning a pregnancy and:

• you've had a child with SMA before
• you have a history of the condition in your family
• your partner has a history of the condition in their family.  

From the Fetal Medicine Centre

In humans, there are 23 types of chromosomes and most people have a pair of each one of these chromosomes (therefore a total of 46 chromosomes). In trisomy, there are three rather than two of a particular chromosome (total of 47 chromosomes). The most common trisomies are those of chromosomes 21, 18 and 13.  

• Trisomy 21 is found in about 1 in 700 births and the risk increases with maternal age. The condition is associated with intellectual disabilities and some physical defects, most commonly heart abnormalities. The life expectancy is now about 60 years, if survival beyond the neonatal period. 
• Trisomies 18 and 13 are found in about 1 in 7,000 births and the risk increases with maternal age. The conditions are associated with severe mental handicap and several physical defects. Most affected individuals die before or soon after birth and they rarely survive beyond the first year of life.

• The results from the test will generally be available within 5-7 days, sometimes sooner.  
• In about 3% of cases the test does not give a result, resulting in a failed test.  This is due to technical problems with the analysis of the sample and does not suggest that there is a problem with the baby. If you want you can have the test repeated (at no cost) and in most cases the repeat test will show a normal result. It is important to ensure that the fetus is alive and developing normally by ultrasound, especially when the first test fails. 

• If the NIPT test shows that there is a high risk that the fetus has trisomy 21 or 18 or 13 you should consider  confirmation with another test.  CVS (chorionic villus sampling) or amniocentesis are considered definite tests to confirm a chromosome abnormality.  If the ultrasound shows a definite abnormality- such as large nuchal translucency, or structural abnormality- AND the NIPT test is positive, then that also indicates a very high probably of an affected fetus.  
• If the NIPT test shows that there is a low risk (less than 1 in 10,000) that the fetus has trisomy 21 or 18 or 13 it is unlikely that the fetus has one of these defects.

• A standard NIPT test does not provide information on other rare chromosomal abnormalities.  Only the Genome test evaluates all parts of all chromosomes.   If the scan at 11-13 weeks shows a high nuchal translucency (more than 3.5 mm) or major defects, such as exomphalos, holoprosencephaly, heart abnormalities or megacysis, the risk for some rare chromosomal defects is high. In such cases you may choose to have CVS or amniocentesis.
• Nuchal translucency evaluation by ultrasound may be helpful, even when your NIPT test is normal. 
• A high quality ultrasound performed at 18-22 weeks remains the single best prenatal test to ensure the baby and pregnancy are developing normally.  NIPT tests do not provide information on physical defects with normal chromosomes,  such as heart or brain abnormalities and spina bifida, or fetal growth.    All women should have an ultrasound scan at 18-22 weeks to examine the fetal anatomy (and also look at other important areas like the cervix) and should consider a scan at 28-32 weeks to evaluate fetal growth.

Amniocentesis is a very specialized test, usually performed a a genetic test.  It is considered the most accurate method of testing fetal chromosomes (and sometimes other things) but carries a small risk of losing the pregnancy. 

 From the Fetal Medicine Centre

• Amniocentesis involves the examination of cells in the fluid from around the fetus (amniotic fluid).
  The cells in the amniotic fluid originate from the baby and so the chromosomes present in these cells are the same as those of the baby.

• Amniocentesis involves passing a thin needle into the uterus in order to remove a small volume of amniotic fluid. The needle is carefully observed using ultrasound scan.
• The fluid is fetal urine and the amount removed by amniocentesis reaccumulates within a few hours.
• The procedure lasts 1 minute and afterwards we check that the fetal heart beat is normal.

• For the first couple of days you may experience some abdominal discomfort or period-like pain. You may find it helpful to take simple painkillers like paracetamol.
• If there is a lot of pain, bleeding, loss of fluid from your vagina or if you develop a temperature please seek medical advice.

• The results for Down's syndrome and other major chromosomal defects are usually available within 3 days. The results for rare defects take 2 weeks. As soon as we get the results, we will call you to let you know

• The risk of miscarriage due to amniocentesis is about 1% and this is the same as the risk from chorion villus sampling. If you were to miscarry due to the test, this would happen within the next five days.
• Some studies have shown that when amniocentesis is performed before 16 weeks there is a small risk of the baby developing club feet. To avoid this risk we never perform amniocentesis before 16 weeks.

Choronic Villus sampling is an alternative specialized genetic test,  similar to amnioctensis, but it can be performed earlier.  Instead of sampling the fluid around the baby, CVS samples the placenta (which forms from the same DNA that makes the baby)

 From the Fetal Medicine Centre

• Chorion villus sampling (CVS) involves the examination of chorionic villi (placental tissue). Both the baby and placenta (afterbirth) originate from the same cell and so the chromosomes present in the cells of the placenta are the same as those of the baby.

• Local anaesthetic is given. A fine needle is then passed through the mother's abdomen and a sample of villi is taken. The needle is carefully observed using ultrasound scan.
• The procedure lasts 1 minute and afterwards we check that the fetal heart beat is normal.

• For the first couple of days you may experience some abdominal discomfort, period-like pain or a little bleeding. These are relatively common and in the vast majority of cases the pregnancy continues without any problems.
• You may find it helpful to take simple painkillers like paracetamol.
  If there is a lot of pain or bleeding or if you develop a temperature please seek medical advice

• The results for Down's syndrome and other major chromosomal defects are usually available within 3 days. The results for rare defects take 2 weeks. As soon as we get the results, we will call you to let you know.

• In approximately 1% of cases the invasive test will need to be repeated because the results are inconclusive.

• The risk of miscarriage due to CVS is about 1% and this is the same as the risk from amniocentesis at 16 weeks. If you were to miscarry due to the test, this would happen within the next five days.
• Some studies have shown that when CVS is performed before 10 weeks there is a small risk of abnormality in the baby's fingers and/or toes. To avoid this risk we never perform CVS before 11 weeks.