Handbook of Genetic Counseling/Advanced Maternal Age - Chorionic Villus Sampling (CVS)-3

Advanced Maternal Age - Chorionic Villus Sampling

Contracting

• Introductions and small talk
• Confirm referring obstetrician
• Assess understanding for the referral
• Elicit concerns and questions
• As maternal age increases, we know that the risk for having a child born with a chromosome abnormality also increases.
• The reason that we like to see women who are 35 or older is because we can offer testing options
• I would like to start today by gathering some medical and family history information so that we can look for any other risk factors that we may need to talk about.
  • We want to see if there is anything in your history that could pose a risk to this pregnancy

Medical/Family History

• LMP:_________
• EDC:_________
• Today's gestational age:_________
• How has this pregnancy been going?
• Any complications?
• Have you had an ultrasound?
  • Date ________
  • Findings:______________
• Anyone in the family have:
  • Birth defects, SABs, SB, MR, LD, chronic illness, or early cancers?
• Tell them what I found; "looks good, it sounds like the pregnancy is going well, I don't see anything that concerns me"*
• Do you have any questions?

Explanations of Genes and Chromosomes

• All pregnancies are at a 3-5% risk of having a baby born with a birth defect.
• Women are born with all the eggs that we will ever have, unlike men who continuously make sperm from puberty throughout their lives.
• Eggs are stopped in a stage of division until one is ovulated each month.
• The older the egg, the more likely a division error might occur.
• General
• Karyotype
• Nondysjunction
• Brief natural history of chromosome abnormalities

Age Related Risks

• Patient's age at delivery: _______
• Risk for Down Syndrome: 1 in ________ ( %)
• Risk for any chrom. Abnormality: 1 in ________ ( %)

Options

• Level II Ultrasound:
  • High resolution ultrasound
  • Done at 20+ weeks by an experienced technician
    • Better to see more of the developing organs
  • U/S detection rate for DS is about 60% and for trisomy 18 ~90%
  • Look for:
    • Heart
    • Brain
    • Shape of the head
    • Intestinal problems
    • Kidneys
    • U/S helps make us suspicious but doesn't give us a yes or no answer.

CVS as an Option

• Offered between 10-12 weeks
• Procedure used to obtain a sample of the placenta from the womb of an expectant mother.
• Genetic make-up of placental cells is essentially the same at the fetus since they both come from the same original cell created at conception
  • When baby is created, the egg splits into two parts: a part that forms the baby and a part that forms the placenta
• Analysis can detect many genetic disorders/chromosome abnormalities with 98-99% accuracy
  • Cannot detect about 90% of birth defects or the severity of a condition

Explain Procedure

• Transcervical CVS
  • Only performed from 10-12 weeks
  • Preferred method if the placenta is posterior and close to the cervix
  • Sterile speculum is inserted and the vagina and cervix are cleaned with betadeine
  • By U/S guidance, a thin catheter is guided into the placenta
  • A syringe is attached and gentle suction is applied to aspirate the villi
  • Large sample (whole villi) taken
  • Catheter is removed
  • NOT recommended for women with a retroverted uterus
• Transabdominal CVS
  • Easiest to perform with anterior or fundal placenta
  • Abdomen is cleaned with betadeine
  • A local anesthetic is applied to the skin
  • By U/S guidance, a needle is guided through the abdominal wall and uterine wall into the placenta (avoid the amniotic sac)
  • A syringe is attached and gentle suction is applied by the syringe plunger
  • The needle is moved back and forth with re-direction through the placenta
  • Small sample (pieces of villi) taken
• Procedure takes roughly 5-7 min, after prep time
• The sample is examined under the microscope to confirm that fetal tissue has been obtained. (If maternal tissue is present- repeat test).
• Following the procedure, the baby's heartbeat will be monitored by U/S
• Sample is sent to the lab for results
• Direct vs. Culture results:
  • Direct analysis: A preliminary answer
    • Analysis of outer cells which are spontaneously dividing
    • Increased rate of error
    • Results in 1-2 days
    • Few labs do this

Culture analysis

• Analysis of cells from mesenchymal core
• More accurate
• Results in 7-14 days
• Results differ in about 2% of cases
• Results will be reported by ????

Risks of complications

• Miscarriage: increased above the background by 1%
• Transverse limb reduction defects
  • Results show that the risk for transverse defects after CVS is ~1 in 3000 (0.03%)
  • Results show the greatest defects when CVS was performed prior to 10 weeks (limbs are formed at 10 weeks)
  • How? Suspected that instrumentation interrupts the vascular supply
  • VUMC has not had any CVS patients who had pregnancies born with limb reductions defects as a result of the procedure.

Disadvantages

• Risk of laboratory failure
• Maternal cell contamination
• Mosaicism
  • Presence of two or more cell lines that differ with respect to their chromosomal constitution
  • Although the placenta and fetus start out the same, once they diverge- there is a very small chance that the placental cells might have a change that is not changed in the fetal cells - confined placental mosaicism
• Insufficient sample obtained
• With any of these, follow up is required.
  • Additional testing

Amniocentesis

• Also an option, performed from 15+ weeks
• Transabdominal removal of amniotic fluid
• ~ same accuracy, but risks are 1 in 200 for complications.
• Tests for NTD's

Follow-up

• How do you feel about having a CVS? Would you like a few minutes to discuss the option with your partner?
• Re-iterate that most babies are born healthy
• Will want to do an u/s at about 18-20 weeks- anatomy is big enough to be easily visualized. (looking for structural anomalies)
• MSAFP blood test (to detect NTD's)
• Elicit final questions and concerns.

Notes

The information in this outline was last updated in 2001.